After adding a member to a user-defined struct type and changing the member type to a user-defined enumeration, repeated edits to that enumeration will eventually cause the editor to become increasingly unresponsive for extended periods of time after each change.
It's easiest to reproduce by repeatedly changing the order of entries, but exposing the "Advanced" settings and repeatedly toggling the "Bitmask Flags" option on/off will eventually result in the same behavior.
This issue can be amplified if the enum is referenced by multiple user-defined struct types.
Note: renaming an enum entry does not trigger this behavior because that type of change does not also cause the referencing struct type(s) to be regenerated.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-115927 in the post.