One of my mac started becoming intermittently unresponsive, so much so that I couldn't even manage to get it to restart regularly. After a hard reset, the pre-boot authentication screen of filevault 2 came up as usual, but would then get stuck (I gave up waiting after 20 minutes) on the screen with the logo and spinning wheel.
Booting from an external disk, I tried unlocking and mounting the disk via Disk Utility - the unlock took about 10 minutes during which Disk Utility mostly appeared unresponsive (rotating colored wheel, red status in the activity monitor).
Eventually the disk appeared in the Finder, and I discovered that navigating into a folder can take about 1-2 minutes! Surprisingly, when I tried copying files that went extremely fast (just as normal I'd say).
How would I go about fixing this problem? (I really wish to avoid reinstalling the machine, for various reasons)