Previous attempts looked at the heaviest query in terms of processing time, IO, or execution count. Those certainly slowed the server down, but the really issue were application freezes of many seconds and even minutes. Fixing those unblocked the server. The heavy queries are still there, but no one cares.

More technically, before, people focused on individual queries with the hope of finding the one that is slowing down the server the most. When we instead focused on sessions, and which ones block others due to locks and DB transactions, we were able to fix the root causes.

I am simplifying a bit, but that is the jist.