Remove plans page load takes more than 2 minutes for 6000+ plans

Issue Summary

As instance administrator I can't access Remove plans page if Bamboo instance has more than 6000+ plans. Loading takes more than 2 minutes and then load balancer returns 504 Gateway time-out error

Steps to Reproduce

1. Create 6000+ plans
2. Open page <BAMBOO_URL>/admin/deleteBuilds!doDefault.action

Expected Results

Page is opened and plans for removal are listed.

Actual Results

Gateway time-out error page

Workaround

Use REST API to remove plans in bulk