Ok, here is the issue with Piklist admin Javascript and there are 2 major issue scenarios.

Tested on default Twenty Sixteen WP theme with default post type “Post” and also custom post types on powerful Macbook Pro (all major browsers)

1. In first scenario we have a meta box “Gallery” containing an add_more field group with file upload and simple stuff (code bellow): First time post is created, you can add as many add_more as you want. As soon as you save the post, the more you have add_mores, the slower the browser responds. if you reach 40 add_mores then 2 issues: takes 15 seconds and more for the page to load – if a new add_more is added or removed when “minus” or “plus” buttons are clicked same thing, 15 seconds or more delay…

2. In second scenario we have the same previous meta box “Gallery” containing add_more folder_content_group field group COUPLED with another meta box “Gallery Folder Display Setting” containing file upload field and some conditionals with NO add_more: Same behavior on first save, no problem with as many add_mores as you want but gets very slow if you pass 25 add_mores and when you reach 40 and up, browser becomes unresponsive and Firefox even prompts to kill the script. Can take 60 seconds for browser to respond on page load and same amount of time just to add an add_more or remove it… Oh, yes, and some HEATING AND SUPER HIGH CPU usage on MacBook Pro (Retina, 15-inch, Mid 2015) – 2.5 GHz Intel Core i7 – 16 GB 1600 MHz DDR3

I’ve seen that some tickets have been opened regarding performance issues with Javascript with and field combination… I was not satisfied by the solutions offered which basically asked to simplify the fields and that it was a lot to handle by for the scripts…

This is for me a major Javascript optimization issue (and maybe also an architectural issue in the way JS has to prepare the html form input fields for PHP handling???)

Because Piklist is meant for developers I do think this needs to be addressed seriously and before the release of the stable version.

Many page builders handle add_mores efficiently, on WordPress or on many many other platforms.

Personally this is an issue because on of my future projects was to create a Bootstrap and Curb Foundation page builder for WordPress using piklist, but I see this is not an option at this time…

Thank you Jason for looking at this. Here are the two different meta boxes. Coupling them on the same post type will certainly allow to duplicate all this.

Let me know if you want me to transfer this to another topic.
Cheers!

Gallery meta box:

<?php
/*
Title: <h3>Gallery</h3>
Post Type: post
Order: 20
*/

piklist('field', array(
    'type' => 'group'
,'field' => 'folder_content_group'
,'label' => __('Gallery Photo','stopping-the-world-admin')
,'description' => __('Add photos to this folder.', 'stopping-the-world-admin')
,'help' => __('Adds a new photo to this gallery folder and sets its availability on the store.','stopping-the-world-admin')
,'add_more' => true
,'fields' => array(
        array(
            'type' => 'file'
        ,'field' => 'single_photo'
        ,'label' => __('Photo','stopping-the-world-admin')
        ,'options' => array(
            'modal_title' => __('Add photo','stopping-the-world-admin')
        ,'button' => __('Add','stopping-the-world-admin')
        )
        ,'required' => true
        ,'columns' => 6
        ,'validate' => array(
            array(
                'type' => 'limit',
                'options' => array(
                    'min' => 1,
                    'max' => 1
                )
            )
        ,array(
                'type' => 'image'
            )
        )
        )
    ,array(
            'type' => 'text'
        ,'field' => 'photo_title'
        ,'label' => __('Title','stopping-the-world-admin')
        ,'help' => __('Sets the photo title. This will appear bellow the photo and above the description on the Gallery Foolder post.','stopping-the-world-admin')
        ,'columns' => 6
        ,'attributes' => array(
                'placeholder' => __('Enter photo title here...', 'stopping-the-world-admin')
            )
        )
    ,array(
            'type' => 'checkbox'
        ,'field' => 'store_availability'
        ,'label' => __('Store Availability','stopping-the-world-admin')
        ,'help' => __('(Leave unchecked if you do not want to sell this photo.)','stopping-the-world-admin')
        ,'columns' => 6
        ,'choices' => array(
                'option1' => 'Stock'
            ,'option2' => 'Print'
            )
        )
    ,array(
            'type' => 'textarea'
        ,'field' => 'photo_description'
        ,'label' => __('Description','stopping-the-world-admin')
        ,'help' => __('Sets the photo description. This will appear bellow the photo and its title on the Gallery Folder post.','stopping-the-world-admin')
        ,'columns' => 6
        ,'attributes' => array(
                'rows' => 5
            ,'cols' => 60
            ,'placeholder' => __('Describe this photo...', 'stopping-the-world-admin')
            ,'maxlength' => '320'
            )
        )
    )
));

Gallery Folder Display Setting meta box:

<?php
/*
Title: <h3>Gallery Folder Display Setting</h3>
Post Type: post
Order: 10
*/

piklist('field', array(
    'type' => 'file'
,'field' => 'folder_cover_photo'
,'label' => __('Folder Cover Photo','stopping-the-world-admin')
,'help' => __('Sets the folder folder cover photo of the folder next to the folder title & description on the Galleries Menu page.','stopping-the-world-admin')
,'options' => array(
        'modal_title' => __('Set folder cover photo','stopping-the-world-admin')
    ,'button' => __('Set','stopping-the-world-admin')
    )
,'required' => true
,'columns' => 12
,'validate' => array(
        array(
            'type' => 'limit',
            'options' => array(
                'min' => 1,
                'max' => 1
            )
        )
    ,array(
            'type' => 'image'
        )
    )
));

piklist('field', array(
    'type' => 'text'
,'field' => 'folder_title'
,'label' => __('Folder Title','stopping-the-world-admin')
,'description' => __('(Max 60 characters.)', 'stopping-the-world-admin')
,'help' => __('Sets the folder title. This will appear above the folder description and next to the folder cover photo on the Gallery page.','stopping-the-world-admin')
,'required' => true
,'columns' => 12
,'attributes' => array(
        'rows' => 4
    ,'cols' => 20
    ,'placeholder' => 'Enter folder title here...'
    ,'maxlength' => '60'
    ,'class' => 'centered-text'
    )
));

piklist('field', array(
    'type' => 'textarea'
,'field' => 'folder_description'
,'label' => __('Folder Description','stopping-the-world-admin')
,'description' => __('(Max 220 characters.)', 'stopping-the-world-admin')
,'help' => __('Sets the folder description. This will appear bellow the folder title and next to the folder cover photo on the Gallery page.','stopping-the-world-admin')
,'columns' => 12
,'attributes' => array(
        'rows' => 5
    ,'cols' => 50
    ,'placeholder' => __('Describe this gallery folder...', 'stopping-the-world-admin')
    ,'maxlength' => '220'
    )
));

// Register checkbox that will control conditionally the displaying of the CSS settings options below
piklist('field', array(
    'type' => 'radio'
,'field' => 'custom_css'
,'label' => 'Customize CSS'
,'help' => __('Check this box to adjust some styling parameters on the folder\'s title, description and containing box.', 'stopping-the-world-admin')
,'columns' => 12
,'attributes' => array(
        'class' => 'text custom-folder-info'
    )
,'choices' => array(
        'no' => __('No', 'stopping-the-world-admin')
    ,'yes' => __('Yes', 'stopping-the-world-admin')
    )
,'value' => 'no'
));

// Register the folder info box field group
piklist('field', array(
    'type' => 'group'
,'field' => 'folder_info_box_css'
,'label' => '<span class="mnl-setting-tab-inner-title">' . __('Folder Info Box CSS','stopping-the-world-admin') . '</span>'
,'fields' => array(
        array( // Register the info spacing field
            'type' => 'text'
        ,'field' => 'folder_info_spacing'
        ,'label' => __('Spacing between the Title and the Description', 'stopping-the-world-admin')
        ,'help' => __('Sets the space between the Folder Title and its Description which appear next to the folder\'s Cover Photo on the Galleries Menu pages. The margin CSS unit used here will be a percentage of the width of the containing box to ensure mobile responsiveness.', 'stopping-the-world-admin')
        ,'attributes' => array(
            'class' => 'number'
        ,'placeholder' => '1.3'
        )
        ,'validate' => array(
            array(
                'type' => 'range'
            ,'options' => array(
                'min' => 0
            ,'max' => 25
            )
            )
        )
        )
    )
,'conditions' => array(
        array(
            'field' => 'custom_css'
        ,'value' => 'yes'
        )
    )
));

// Register the folder title field group
piklist('field', array(
    'type' => 'group'
,'field' => 'folder_title_css'
,'label' => '<span class="mnl-setting-tab-inner-title">' . __('Folder Title CSS','stopping-the-world-admin') . '</span>'
,'fields' => array(
        array( // Register the title font size field
            'type' => 'text'
        ,'field' => 'font_size'
        ,'label' => __('Font Size <span class="mnl-label-comment">(4 decimals max.)</span>', 'stopping-the-world-admin')
        ,'help' => 'Sets the font size of the Folder Title which appear next to the folder\'s Cover Photo on the Galleries Menu pages. The CSS unit used here will be in \'em\' and thus will depend on the parent element font size.'
        ,'columns' => 12
        ,'attributes' => array(
            'class' => 'number'
        ,'placeholder' => '1.525'
        )
        ,'validate' => array(
            array(
                'type' => 'range'
            ,'options' => array(
                'min' => 0
            ,'max' => 25
            )
            )
        )
        )
    ,array( // Register the title line height field
            'type' => 'text'
        ,'field' => 'line_height'
        ,'label' => __('Line Height <span class="mnl-label-comment">(3 decimals max.)</span>', 'stopping-the-world-admin')
        ,'help' => 'Sets the line height of the Folder Title which appear next to the folder\'s Cover Photo on the Galleries Menu pages. No CSS unit is used here.'
        ,'columns' => 12
        ,'attributes' => array(
                'class' => 'number'
            ,'placeholder' => '1.5'
            )
        ,'validate' => array(
                array(
                    'type' => 'range'
                ,'options' => array(
                    'min' => 0
                ,'max' => 25
                )
                )
            )
        )
    ,array( // Register the title letter spacing field
            'type' => 'text'
        ,'field' => 'letter_spacing'
        ,'label' => __('Letter spacing <span class="mnl-label-comment">(3 decimals max.)</span>', 'stopping-the-world-admin')
        ,'help' => 'Sets the letter spacing of the Folder Title which appear next to the folder\'s Cover Photo on the Galleries Menu pages. The CSS unit used here will be in \'em\' and thus will depend on the parent element font size.'
        ,'columns' => 12
        ,'attributes' => array(
                'class' => 'number'
            ,'placeholder' => '.1'
            )
        ,'validate' => array(
                array(
                    'type' => 'range'
                ,'options' => array(
                    'min' => 0
                ,'max' => 10
                )
                )
            )
        )
    )
,'conditions' => array(
        array(
            'field' => 'custom_css'
        ,'value' => 'yes'
        )
    )
));

// Register the folder description field group
piklist('field', array(
    'type' => 'group'
,'field' => 'folder_desc_css'
,'label' => '<span class="mnl-setting-tab-inner-title">' . __('Folder Description CSS','stopping-the-world-admin') . '</span>'
,'fields' => array(
        array( // Register the description font size field
            'type' => 'text'
        ,'field' => 'font_size'
        ,'label' => __('Font Size <span class="mnl-label-comment">(4 decimals max.)</span>', 'stopping-the-world-admin')
        ,'help' => 'Sets the font size of the Folder description which appear next to the folder\'s Cover Photo on the Galleries Menu pages. The CSS unit used here will be in \'em\' and thus will depend on the parent element font size.'
        ,'columns' => 12
        ,'attributes' => array(
            'class' => 'number'
        ,'placeholder' => '.08'
        )
        ,'validate' => array(
            array(
                'type' => 'range'
            ,'options' => array(
                'min' => 0
            ,'max' => 25
            )
            )
        )
        )
    ,array( // Register the description line height field
            'type' => 'text'
        ,'field' => 'line_height'
        ,'label' => __('Line Height <span class="mnl-label-comment">(3 decimals max.)</span>', 'stopping-the-world-admin')
        ,'help' => 'Sets the line height of the Folder description which appear next to the folder\'s Cover Photo on the Galleries Menu pages. No CSS unit is used here.'
        ,'columns' => 12
        ,'attributes' => array(
                'class' => 'number'
            ,'placeholder' => '1.5'
            )
        ,'validate' => array(
                array(
                    'type' => 'range'
                ,'options' => array(
                    'min' => 0
                ,'max' => 25
                )
                )
            )
        )
    ,array( // Register the description letter spacing field
            'type' => 'text'
        ,'field' => 'letter_spacing'
        ,'label' => __('Letter spacing <span class="mnl-label-comment">(3 decimals max.)</span>', 'stopping-the-world-admin')
        ,'help' => 'Sets the letter spacing of the Folder description which appear next to the folder\'s Cover Photo on the Galleries Menu pages. The CSS unit used here will be in \'em\' and thus will depend on the parent element font size.'
        ,'columns' => 12
        ,'attributes' => array(
                'class' => 'number'
            ,'placeholder' => '.1'
            )
        ,'validate' => array(
                array(
                    'type' => 'range'
                ,'options' => array(
                    'min' => 0
                ,'max' => 10
                )
                )
            )
        )
    )
,'conditions' => array(
        array(
            'field' => 'custom_css'
        ,'value' => 'yes'
        )
    )
));

Here is a temporary ugly fix I made in case somebody needs it while you guys are hopefully trying to fix this in core:

Replace of course the value of the MY_CORRUPTED_METADATA_META constant according to your fields

if( ! function_exists( 'hdev_filter_piklist_corrupted_metadata' ) )
{
   /**
    * Define field group fields constant and apply filter
    */
   function hdev_filter_piklist_corrupted_metadata()
   {
      // Define all the meta keys we want to process here as an array constant [string META_KEY => array FIELDS] (make sure to include all the groups fields in the right order)
      define( 'MY_CORRUPTED_METADATA_META', array( 'folder_content_group' => array( 'folder_title', 'single_photo' ) ) );

      // Hook filter
      add_filter( 'update_post_metadata', 'hdev_overwrite_piklist_corrupted_metabox_fields', 10, 5 );
   }
}
add_action( 'init', 'hdev_filter_piklist_corrupted_metadata' );

if( ! function_exists( 'hdev_overwrite_piklist_corrupted_metabox_fields' ) )
{
   /**
    * Overwrite Piklist corrupted post metadata with correct value before it is added to the database
    *
    * @param $null
    * @param $object_id
    * @param $meta_key
    * @param $meta_value
    * @param $prev_value
    * @return bool|null
    */
   function hdev_overwrite_piklist_corrupted_metabox_fields( $null, $object_id, $meta_key, $meta_value, $prev_value ) {

      // Loop through meta keys associated with potential corruptible metadata
      foreach( MY_CORRUPTED_METADATA_META as $corrupted_meta_key => $corrupted_metadata_fields )
      {
         // Possible correct data if processing potential corruptible metadata
         if ( $corrupted_meta_key == $meta_key )
         {
            // do nothing and continue normal execution if new value is empty
            if (  empty( $meta_value ) ) {
               return null;
            }

            // Initialize new meta value var
            $corrected_meta_value = array();

            // Initialize key count
            $key = 0;

            // Loop through all the filed group fields defined earlier in the constant
            foreach( $corrupted_metadata_fields as $corrupted_metadata_field )
            {
               // Make sure the new meta value is corrupted, correct if necessary
               if ( ! array_key_exists( $corrupted_metadata_field, $meta_value ) )
               {
                  // Correct
                  $corrected_meta_value[$corrupted_metadata_field] = $meta_value[$key];
                  $key++;
               }
               else
               {
                  // Do not correct
                  $corrected_meta_value[$corrupted_metadata_field] = $meta_value[$corrupted_metadata_field];
                  $key++;
               }
            }

            // continue normal execution if meta value was not corrupted
            if( $corrected_meta_value == $meta_key ) {
               return null;
            }

            // Unhook just in case
            remove_filter( 'update_post_metadata', 'hdev_overwrite_piklist_corrupted_metabox_fields', 10 );

            update_post_meta( $object_id, $corrupted_meta_key, $corrected_meta_value );

            // Reapply hook
            add_filter( 'update_post_metadata', 'hdev_overwrite_piklist_corrupted_metabox_fields', 10, 5 );

            return true; // Stop normal execution
         }
      }

      return null; // continue normal execution
   }
}

I noticed this bug while testing Piklist 0.9.9.2 by the way, did not test on other versions