Just to add more detail, in case it helps diagnose… I’ve been experimenting further, removing elements and testing, and it appears that what is causing the slowdown (which increases with the number of repeated elements) is the conditionals – and the slowdown increases with the number of conditionals. I’ve for now removed these, and there’s no slowdown, but of course it can’t act as a flexible content block (a la ACF) if you can’t choose between sets of fields.

Anyway, it’s a great library, I’m enjoying using it – hopefully the conditionals issue can be fixed in a later release (if there isn’t already a way round it?)