When i reach more than 300 elements (i have around 700 meta) in a custom taxonomy, to add or to edit a Field group make a freeze (or a really really slow loading) of WordPress (just when adding or editing a metabox custom fields).
I guess this is because of the taxo loading for taxo fields type.
If you want to add more fields to the custom taxonomy, I recommend using the code to add/edit fields. We will try to research and improve the Builder to load more fields with the least time.
Hi if I understand correctly, I also have this issue.
I believe OP means "more than 300 terms in a (any) custom taxonomy"
And when editing (or creating a new field group) it loads very slow.
Presumably because it's loading those terms for taxonomy-type fields, but this happens even when no taxonomy (or any) fields have been added to the field group.
We have the plan to rebuild the MB Builder by React to reduce the loading time when creating the field group. Please wait for the big update in the next time.
The developer team is working to create a new version of the MB Builder. You can try to use the Online Generator tool which has been implemented some React JS code https://metabox.io/online-generator/.