Started mongod in windows and ran manyConnectionsTest.py from my mac in 3 different shells.
Read and write performance would initially fluctuate but eventually level off a couple minutes after all client threads were up and running. There would then periodically be periods of time where the read and write rate fluctuated wildly, before eventually re-stabilizing with worse performance. After a few of those, the periods of performance fluctuation would go away, and instead the performance would degrade gradually.

For one such run, performance initially stabilized at roughly 2000 reads/sec and 180 writes/sec. After a period of fluctuating read and write rates, it re-stabilized at 2000 reads/sec with only 128 writes/sec. After another burst of unstable performance, it re-stabilized at roughly 700 reads/sec and 83 writes/sec. Then there was no more great variability in performance - instead the write rate went slowly but steadily down as the read rate slowly went up. I checked in periodically and saw QPSs of 718 reads/sec w/ 61 writes/sec, 733 reads/sec w/ 46 writes/sec, 755 reads/sec w/ 23 writes/sec, and 763 reads/sec w/ 163 writes/sec. At around this point, performance suddenly fell away to zero.

At this point, the python processes running manyConnectionsTest froze, printing an error message saying the find_one operation timed out.

At this point, however, it was still possible to create new connections to the mongod, and it could process queries from those new connections.