Actually it's not. Reason is that it has so much to do or internet connection is slow.

Install Faster Application Manager to speed up app install and remove apps. Remove apps that you dont need.

And then install terminal program htop to easily see what is going on inside N900:
- CPU shows how much processor is in use normally its under 25%, so you don't have to overclock. Some Easy Debian programs start very slowly, but after they started they don't need so much CPU power.
- Mem shows how much memory is in use. Normally it uses under 200 MB from 229MB, but that doesn't tell all, next thing tell if there is problem with memory use.
- Swp shows how much doesn´t fit to memory and is putted to slower swap memory. Under 100MB from 767MB is normal. If this go over 300MB then I have noticed that N900 get slower. If you have to have many heavy web pages open at once and you feel slow responses try to Opera Mobile.
-Tasks shows how many processes there is running. Normaly it is under 150. If it show over 175 then there can be some ghost processes and then you should restart your phone.
- Load avarage is process load avarage
- Uptime is how long time your N900 have been on after restart. If this is many weeks then you can try restart to speed up things.