For me, OCing helped a lot. getting it to 850 Mhz really showed a performance boost.

Are these FEELED 30 secs or REAL 30 Secs.
Cause when i just changed from my C2Q X9650 with 8 GB Ram my N900 may eventually feel a bit slow.

Regarding firefox, the Firefox does some strange things on startup, depending on your settings. Like reading your bookmarks, so, if you use firefox sync to sync your 6 billion Bookmarks, well, maybe the startup will be slow.

And some Pages in the Net are REALLY eating up ressources. iGoogle can (depending on your config there) eat up lots of ressources. There are some more tung options, search for tuning and swappines.