Create plan with large amount of tests, for example : 35000
Run few builds when these tests pass or fail.
When open Summary of this plan Bamboo tries to load all these test cases from DB into memory and display only 50 of them.
At the same time when DB connection is taken by test-related query other queries don't processed - fetching chain details etc.
Also for each test we fetch last success build number. Not optimal to do it in record by record

On MySQL DB may experience deadlocks

Workaround

Follow the steps in the following KB article: Build results summary pages are slow or unresponsive