I just bought a USB female-female gender changer to plug my USB keyboard in on one end and a USB to micro USB cable leading to my N800 on the other end. I hooked everything up, typed "echo host > /sys/devices/platform/musb_hdrc/mode" into the N800's terminal, and everything works perfectly! No lag whatsoever, neither with text mode apps nor with Hildon/X apps, and I"m a very fast typist. Didn't even have to use a powered USB hub as some others were claiming.

So, this means that the lag has to do with bluetooth somehow. I tried building a new version of bluez, but that requires a new version of glib. Believe it or not, I'm almost finished porting glib 2.30 to Diablo (yes, including Nokia's Hildon-related changes), but it needs a new version of gettext to build.

EDIT: The USB keyboard also doesn't rrrrrrrrrrrrrrepeat letters the way that Bluetooth does occasionally, but it still has the messed up layout where an apostrophe becomes a grave accent. That still needs to be fixed by xmodmap, etc.