Just want to add my 2 cents: how your N900 performs largely depends on what is run, how it is tuned, and how you optimize the system havig unsatifactory factory settings by Nokia.

In my case the most frequent and prominent lagging happens when running several brower windows, terminal sessions, AND that wtf HAM.

Other than that,
Mine runs nicely, though not *perfectly*, contant@1GHz having 7 or 8 windows opened, including games.
IMHO lagging happens because of various factors. One simple way to catch the naughty app is by looking at the CPU & RAM usage. Conky, applets, (h)top, etc. will give you a clear view.
I am thinking of some really bad guys and what I do about them:
- HAM: n times slower than FAM. (Almost) hang up even when installing just one small app. Takes ages to refresh and installing, regardless how fast you OC and without running any other applications. It is a real piece of crap, even worse than the app manager in windows mobile.
Remedy: Use FAM, or even better and faster just apt-get (which I do 90% of the time). And like others have said. Send the update checking task to oblivion.
- Tracker: uses a lot of resources when desktop starts up.
Remedy: tracker-cfg
- Various daemons: some may install and run "secretly" without being noticed after installing certain apps.
Remedy: ps or whatever to find them out. Kill them and remove those packages if possible
- Frequency: after OC I don't think I would go back to the default setting that sucks.
Remedy: OC w/ some tuned presets, e.g. those from Queen Beecon Widgets
- Swapping
Remedy use Swappolube or BoostN900.
- Crowded and slow desktops: it's an overkill to have many widgets working busily (updating, etc.) on the desktops.
Remedy: Use only those widgets you really need, use less fancy decorations, use Matan's modified-hildon-desktop, etc..

This phone, just like all others, inevitably leaves something to be desired.
The nicest thing about this phone is we have very frequent releases of new fixes, new kernels, new modified apps and even new alternative OSs and booting methods.
The wiki has MANY more tricks and goodies to improve the user experience, e.g.
http://wiki.maemo.org/Customizing_Maemo
http://wiki.maemo.org/User:Joerg_rw/tools
http://wiki.maemo.org/Remapping_keyboard
etc. etc.

Last but not least, being able to run full scale distros like Debian well indicates that the phone is not crap compared to other not-so-smartphones.