you could try a reinstall of the emmc and the firmware you could try overclocking to say 700mhz or 800mhz but this will void your warranty have you got devil apps installed etc there are alot of reasons your n900 may have slowed down , just think your pc slows down sometimes and the n900 is like a mini pc
Have you tried to switch it off and then on? - Switch off - Take battery off - Put i back and restart If it's still slow then - Start xTerminal from more - Run command top - Report here is something is on top with high cpu (over 10% of cpu, xorg should be 1-3% its normal) Also please tell us. Have you installed anything from extras testing or development?