Yesterday and the day before my machine was on it’s knees. I wouldn’t respond to anything , be it keyboard or mouse and when it did, it really was very slow. I tried looking at the culprit sucking CPU but couldn’t find it. The culprit looked like it would be either Firefox of Thunderbird. I then noticed that mdworkera process was stuck (thanks to trusty top).
I issued the following command sudo fs_usage -w -f filesys mdworker | egrep “open” and found that Library/Cache/Firefox had plenty of file that where being worked on simultaneously by those spotlight process and Firefox itself.
I then went to System preference , spotlight and added the Library/Cache/Firefox directory to the list of things I didn’t want to be indexed for privacy reasons. My machine’s perf is now back to normal.