hello everybody i just wanted to share one of my experiences, i bought a brand new n900 and installed the power kernel plus some scripts i found in a thread that's supposed to make the phone faster, so i rebooted and yes it did seem a little bit faster, i used it for about a week and then i decided to buy a microsd card, i opted for a kingston class 10 16gb, so i installed the card and after a few hours i was browsing the net and decided to intall some applications and everything was lightning fast even the appmanager witch before was really really slow, practically when i click install from this site the app manager opens and the installation begins immediately where before it would have required 7-8 seconds, everything is faster!

Did you reboot after installing the sd card?? If yes, then it is just that reboot seems to make the n900 faster. After 3 to 4 days it will again slow down.

Check out some of the threads like

swap on microsd
powerkernel - overclocking
speedpatch + tweaks
swappolube
optimizen900

they do give some good performance boost.