Summary

User Picker Dropdown takes an excessive amount of time to load when there are thousands of users in Jira and up to 49 users in the Assignable User permission.
 

Steps to Reproduce

1. In the permission scheme that is being used, set the Assignable User permission to 49 users or less, either using groups, project roles or individual users.
2. With the user picker, whenever a user types a name into the field (assignee, reporter), the auto-complete list populates adequately.
   • However, when a user selects the dropdown to try and assign a user, it encounters a timeout at 1 minute.
   • When using the api call alone, it registers at 1.7 minutes to load. The exact call being used by the dropdown is:

     rest/api/latest/user/assignable/search?username=&projectKeys=[REDACTED]&issueKey=[REDACTED]&maxResults=50&startAt=0&_=1559132212476
     

     This is not something where adjusting the timeout will suffice because in no way should the dropdown timeout and take that long to load.

Expected Results

 It should take less than a second to populate the list.

Actual Results

It takes over 2 minutes for the list to load and at times, it times out. 

Workaround

• There's a good chance it will only take a few seconds for the list to populate by adding 50 users or more to the permission. This will only happen if Jira finds the 50th user relatively quickly.
• If Jira needs to iterate through several batches to find the 50th users then it may still take a very long time. Each search batch goes through 10000 users.
• Until JSWSERVER-20336 is fixed, the workaround that is guaranteed to work is not having any assignable permission and relying on the project browse permission. i.e. Anyone who's able to see an issue can be an assignee.