To be honest, I was feeling the same thing after the firmware update so what I did was reflash the device using the Global variant. Everything was slow and laggy.

After the reflash my N900 has never been smoother and snappier. It's like I have a new phone everyday. I have a lot of extra-devel apps installed and my phone is very smooth

I can't explain how glad I was to see my phone that good after the reflash.

(Btw I reflashed it a week ago and it's still very smooth)