So, where is is broken. Is this on in the admin or the front end? Does the page error or time out? If it’s the front end can you give me an overview of how where and how these fields are loaded? for example what template, is it a custom query? Any additional information? I can tell you that there can be problems with ACF with huge, complex field groups, I’ve run into them myself, but there may be a way to decrease the impact of it.
It’s broken on the back-end. You can click add item to the Repeater field, but the Flexible Content field added has no options in it. It’s just an empty row — no buttons or anything. I dont’ see any errors in the console. Reverting to 5.3.0 works.