Recently, I’d noticed a bunch of cases where MongoDB would be far, far slower to build indexes on secondaries than on the primary. An index build would finish in a few hours on a primary, but then take a day or more to build once the indexing operation replicated to a secondary. Eventually I got annoyed enough to decide to debug. I threw perf and PMP at a build that was running on a secondary, and they mostly just informed me that the build was spending most of its time comparing BSON objec...
