Very slow response on report configuration page with 10K crowd groups

It takes about 10 minutes to open report configuration page of JIRA Timesheet Plugin because of 10K groups in Atlassian Crowd directory connected to JIRA.

It may be a regression, as it's working fine on JIRA 5.0.5 and 5.1.8.

Note, problem is even worse for some customers:

• 20 minutes to open report configuration page here https://bitbucket.org/azhdanov/jiratimesheet/issue/346/jira-timesheet-reports-is-working-realy
• OOM here https://answers.atlassian.com/questions/153863/time-sheet-reports-don-t-open-in-jira-5-2-7)

Steps to reproduce:

Prerequisites:
1) JIRA 5.2.10 (latest)
2) Install Atlassian Crowd (evaluation license is enough) and integrate it with JIRA as described here: https://confluence.atlassian.com/display/CROWD/Integrating+Crowd+with+Atlassian+JIRA
3) Generate 10K groups (e.g. using ruby script: https://bitbucket.org/azhdanov/jiratimesheet/src/default/src/test/ruby/generate-crowd-groups.rb)
4) JIRA Timesheet Plugin installed (latest version): https://marketplace.atlassian.com/plugins/jira-timesheet-plugin

Scenario:
1) Go to Projects -> e.g. Demo project -> Reports -> Time Sheet Report
2) Expected page loaded in 3s or less, but it hangs for 10 minutes or more , with no any response from server, or even OutOfMemory happens.

Note, problem is caused by Group selector, however backing GroupValuesGenerator is out of concerns, because it's used in Timesheet Gadget also, and there is no any problem with response time.