We noticed today that one of our Mongo servers had unexpectedly high CPU usage...which was fixed by restarting.

We were running about 258 ops/s on a Google Compute instance with 12 CPUs + 24GB RAM, with 30% CPU usage. I ended up restarting the instance, and we noticed the next set of ops was MUCH lower - it didn't rise above 3%.

We thought that was odd, so we repeated the test on another mongo server we had running the same code.
1) Run repeatable set of requests against mongo
2) Restart server
3) Run the same set of requests
4) Get drastically lower CPU post-restart.

Any ideas on what might be happening here?