I’ve recently noticed that when using large numbers of fields (nested flex/repeater fields in my case, but not sure that matters), the WP backend becomes increasingly unresponsive while editing posts. After some experimentation I tracked down the biggest bottleneck to editor-expand.min.js, which seems to be monitoring scroll events.
I was able to fix this by removing post type support for the main editor selectively on pages with the field group I was having issues with. At worst it halved script execution time and in some cases almost eliminated it. It does not seem to matter if you uncheck the “Enable full-height editor and distraction-free functionality” option, but removing the editor entirely does the trick. It also doesn’t seem to matter if you have other WYSIWYG editors on the page or not, only the main editor has any visible impact.
It may be possible to fix this in some other way, but if not I’d suggest having ACF remove support for the editor rather than just hiding it when the “Hide Editor” option is checked on a field group. Doing so significantly improves the performance on pages with many fields. This isn’t an ACF bug so far as I can tell, it seems to just be a side effect of an interaction with the WP Core code, and I’m not sure it’s specific to ACF.
