Last modified: 2014-08-14 12:37:18 UTC
It's probably because user_id filter causes filtering of a ton of data in-memory. Pushing the filter into the subqueries should improve performance a lot.
Change 149482 had a related patch set uploaded by Milimetric: Fix slow Rolling Active Editor metric https://gerrit.wikimedia.org/r/149482
Change 149482 merged by jenkins-bot: Fix slow Rolling Active Editor metric https://gerrit.wikimedia.org/r/149482