So I'm still assuming this is related to this topic (sorry if not) but I found the conflict in my case.
- First I tried disabling all plugins except MB & MB AIO. I also tried changing to 2020 theme. My problem persisted: "Edit field group" window completely freezes and uses up 100% of CPU. No errors in the logs.
With WP Debug on, I see only this warning
Notice: Undefined index: taxonomy in /wp-content/plugins/meta-box-aio/vendor/meta-box/mb-custom-post-type/inc/taxonomy/register.php on line 113
But this warning shows with MB AIO 1.10.7 as well, which does not freeze up.
Next I started disabling AIO Extensions 1 by 1. The one that fixed it was disabling "Meta Box Include Exclude". Then my "edit field group" loaded fine. It didnt matter which field group I'm editing (whether it's using include/exclude or not).
I'm just happy I know how to avoid it, as it was locking up my whole site. I hope this can help!