Load time can be improved in the Admin for WYSIWYG editor fields using delayed or interaction triggered loading via a checkbox when using the Classic WYSIWYG Editor field
Delay initialisation?
TinyMCE will not be initialised until field is clicked.
ACF field speed can be also be improved with local json
https://www.advancedcustomfields.com/resources/local-json/
Additionally you may want to explore using Transient caching:
https://developer.wordpress.org/apis/handbook/transients/
Hope this information helps your site performance.
