Hi. I don't think small rootfs will slow down the device. 9mb is dangerously low though. You should uninstall some apps you got from extras-devel and extras-testing, since if rootfs comes down to 0 your device will brick (you'll need to flash it). I think a freshly flashed N900 has about 70mb free rootfs, I personnally run at 58mb free and installed nearly nothing from devel and testing. Hope it helped.