In a workload where explain() is called a lot I've lately seen a certain regression in throughput.
I was able to isolate the commit that caused the problem.
https://github.com/mongodb/mongo/commit/6dd40e54de4d62ed1c649f6f620a574e6de74fa8

The throughput looks like 20% less and I'm completely able to recover if I revert to the previous revision. See screenshot attached.