Ubuntu slows down over time?

First install htop and iotop:

sudo apt-get install htop iotop

Then start three terminals, and run each of these commands in each of them:

iotop

This command will show you any process that is writing to the hard drive

htop

This command will show a more colourful alternative to top. Pay special attention to the amount of free memory. If this starts running low, you will probably soon see an increase in disk writes in the iotop-command (above), which in turn may indicate that something is leaking memory...

tail -f /var/log/messages

This command will show system messages. If some program crashes or such, more info may popup here. So when you start noticing system lags, check the output from this command...

Of course, non of the above actually solves your problem, but it might help diagnose it.

Cheers /N

Your real problem could be overheating. I had this exact same problem. All I did was elevate my laptop by keeping it on Dana's Text Book of MIneralogy (4th Edition). Your problem is overheating if you hear your fan whirring at a high speed. It is actually struggling to keep your laptop cool. Processing power is diverted to keep the fan going.

Check your memory usage - I run the System Monitor applet in my panel, and it lets me know if I'm getting close to running out of memory. When I run too many programs, I have similar problems - really slow responsiveness.

If it is a memory problem, try using top or the full system monitor tool that you can launch from the applet to identify which process(es) are using up your memory. Often you can remove it and not have problems, but double-check first.