Definitely something is really wrong. Even with a very slow class 1 sdcard there is not too much lag and no freeze.
You probably need to start over with no overclock first.
Be sure to extand root on the entire sdcard and check if swap is enabled. Then reboot.
Don't forget that the first time you connect your device on WiFi apt-worker will start automatically and N900 will be very slow for let's say 3-4 minutes.
After a new installation, if your N900 is still freezing and lagging i think your sdcard is probably faulty or corrupted.