Currently GF uses Chosen 0.9.3, which has a very nasty bug in it when you get to a certain # of items in a list. We used the Chosen feature in GF for a tags field on a custom post type with about 600 tags for instance, which caused the browser to freeze up.
You can read about the issue and how it was resolved here:
https://github.com/harvesthq/chosen/issues/275
Please ensure this is included in the next bug fix update - thanks!
