ACF has a delayed initialization for WYSIWYG fields, and I would suggest using this.
There is no solution for async loading of fields.
As for the slow saving. There isn’t much that can really be done here. ACF works by using built in functions for updating post meta fields. ACF has no way of know what content has been changed. The WP function update_post_meta() and equivalent do this work. Basically, because of the way meta values work, this is done 1 field at a time. WP supplies no mechanism for doing it any other way.
I see this as a flaw in WP and not in ACF. ACF’s goal, to my understanding, is to allow us to create an admin for editing custom meta values using what is available to all of us without ACF. The problem is that it’s really easy to create these monolithic admin interfaces without realizing that there may be a problem down the road.
This is an inherent limitation when using WP and repeaters/flex fields. There have been many discussions about this here https://support.advancedcustomfields.com/forums/search?bbp_search=slow+saving. If you read any of these you’ll see that I’ve suffered from the same problems myself. There are no good fixes, anything I’ve done has just been hacks that have had varying degrees of success. The only real solution is to be aware of the limitations and develop in a way that works around those limitations, as you suggest, by breaking things up into smaller chunks. Unfortunately, by the time most people realize that there is a limitation they are already having issues because of the limitation.
