One more thought - often slow computers really are just slow DNS servers. That can make an otherwise fast system, feel slow. There is a tool that will find the fastest (not necessarily the most trusted) DNS server for your location. I ran that a few months ago, found the fastest DNS that didn't rob my privacy, then setup my home router to use it and all the systems here to only point to the router for DNS. The fastest was NOT my ISP, BTW.
Oh - and I have 1 system that uses the fglrx driver. After every new kernel is installed, I manually reinstall that driver like this:
There are other steps, but those can only be performed when X/Windows is NOT running. That means stopping all GUIs - even the login manager, then running some aticonfig commands. Yours will be different from mine, so I won't post them here. RTFM to see the options. The system with the ATI GPU is for a silent media center running Plex and XBMC - fairly specialized.
sudo aptitude --reinstall install fglrx
How is this system connected? If wifi, try wired to see how much that helps. wifi connections can be terrible or great. If it has been awhile, use a wifi analyzer to determine if your neighbors are screwing with your connection. I discovered last night that a close neighbor
had gotten a new router installed that was on the same frequency as mine. Throughput sucked the last few weeks. Need to change to a different channel this morning. 28 or 30 systems here are wired, so the change in performance due to wifi interference really isn't noticeable for the lite-surfing done on those. I expect a 20% performance boost by just changing the channel on the router this morning.
Anyway - gives you some more things to look into.