As I've pointed out in comments #9 and #10 of https://bugs.maemo.org/show_bug.cgi?id=8723 -- this issue happens to me over few days of usage. For example when I reboot device it works "blazingly fast", but after about 3-4 days of uptime I start experiencing this sluggishness each time I wake-up device from sleep. It is getting unbearable, so I've just rebooted my N900 today and wait how things will evolve over next couple of days. Once I start getting this again, I'll start removing widgets one by one from my desktop to try narrow potential suspects down.

My current widgets are: conversations inbox, personal dataplan, personal ip address, media player, fmtx, recaller, touchsearch.