Mongo-sim based microbenchmarks that update a hot document (i.e., all updating same document) show significant performance degradation today. 10 threads trying to update the same document.

Throughput for the two tests
12: 2489 2458 2386 419 375
15: 45328 43026 43455 13261 5895
12 is a table scan based, and 15 is index scan based. Last two numbers are for builds from today(4ab029e3 and c686f5a19 respectively). First three from last week.