When adding large numbers of results to a set in the search/browse results interface it can time out resulting in only some items getting added to the set. The upper bound of # of results after which time outs occur depends upon the server but is typically between 1000 and 2000. Worse, the user has no idea what is going on as the UI provides no feedback until the add operation is complete.
The fix for this issue is:
1. Improve "add items to set" performance to avoid timeouts
2. Add a "busy" spinner to the UI so the user at least knows something is happening.