test_wt2853_perf, when modified to use 2 reader threads and 2 writer threads (or other combinations that aren't merely 1R1W), occasionally shows a performance slowdown after about 40K entries are inserted. The slowdown lasts for several seconds, and then the pace picks up again, and doesn't exhibit the slowdown again.

Background: In WT-5945, I started to do an investigation on this, but it got beyond the scope of that ticket. But anyone working on this should probably read that ticket for more context.

Also, this test case uses WT indices, which MongoDB does not. However WT indices are a pretty thin layer - It wouldn't be too hard to rewrite and simulate the use of indices, and see if that changes anything.