File System seems slow/unresponsive: how to measure/test?
The upgrade might have corrupted your Spotlight index - which would result in the computer constantly reindexing itself (or at least, seeming to). I've heard this is a common occurrence when upgrading through OS X - it happened to me when I upgraded to Lion.
You can destroy the Spotlight index by doing the following:

Go to System Preferences → Spotlight → Privacy and exclude your HD, reboot and then include it again. This should prompt Spotlight to reindex (having destroyed the old index). Source

If that doesn't work, try deleting the Spotlight index manually. This will force it to reindex.

In terminal run man mdutil - this will allow you to verify by using the manual function that this is a command related to spotlight.

Once satisfied run mdutil -i off, switching off the Spotlight index.

Run sudo rm -rf /.Spotlight-V100 This will delete the entire Spotlight cache.
You may prefer to rename it using sudo mv /.Spotlight-V100 /disabledspotlight - it will have the same effect, and gives you a backup.

Run mdutil -i on to restart indexing.

Leave for a few hours while it does its thing.


Some references as to the safety of doing this:

MacRumors Forum Thread: &quot;Is .Spotlight-V100 safe to remove?:
Apple Support Communities Thread: &quot;Removing/deleting Spotlight-V100?&quot;
the Apple Genius that helped me when I was having Spotlight issues.

This could be totally off, and not your problem at all - but on the other hand, it might help.