did you keep all your conversation history since day 1?
don't know about MMS (don't use it) but simply 2+ yrs of convo means some SMS/IM windows take a couple seconds to open.
phone calls are fine, but i have reflashed (even my primary N900) at least twice a year
and i don't use any games / themes

basically, even though i love to have a... Linux computer in my pocket it is 1st and foremost my (mobile) phone.
whenever i install something, this has to keep working. if it doesn't after installing something, something is gone
if something can't be uninstalled properly (CSSU comes to mind ), re-flash...

i have CSSU-thumb on my 2nd N900 (30 to 40 MB more RAM available compared to prime) but CSSU is such as mess...
and i don't do (many) phone calls from nbr.2...