Allow better configuration of User Picker custom field

Introduction

How user picker works

• It figures out the group via Project -> Role -> Group (i.e. determines that the assignable users for project X are the users in the role Y, and therefore the users in the group Z).
• Uses the configured User Directories (AD, Crowd...) to get a list of users that belong to group Z.
• Matches the entered characters with the user name, the email address and display name.

Can you change the interval
Unfortunately no. It will do a request 75 millisecond after entering a character. If the user enters another character in that 75ms period, it will restart the timer. In other words, if the user types the name really fast, it won't do a search until the full name has been entered. A hacky way to force that behaviour is to write the username somewhere else, copy and paste it into the field: that will result in just one REST call.

Will it cause performance issue
Yes, It could cause performance problems. At least, there are a two potential problems:

• If the user has multiple User Directories, searching for the members of a group could be slow. Solution: reduce the number of User Directories and/or move the User Directory that will contain the requested group at the top of the chain.
• If the project has a very broad assignable group, (eg: all the users in JIRA are potentially assignable, but only a bunch of them are actually assigned), retrieving the list of users and matching the characters could be expensive. Solution: create a smaller assignable group, it is better to add users when needed, than to add lots of JIRA users that never get an issue assigned.

Suggestion

It would be great if JIRA administrators can configure User Picker, for example the REST interval.