I'm running a custom 6.5 rom on the 21501 build and I used to have lag. I found a solution while trying to troubleshoot my GBA emulator.
I too, at first, believed that is was the display that was lagging the input of text. However, delving further into it I have found that it is how winmo interprets key codes coming from hardware keyboard.
Winmo hardware keyboards typically come with a modified msim*.dll which tends to be labeled something like msim*.dll where * is some identifier with digits. The reg code that points to the file is:
[HKLM\System\CurrentControlSet\Control\Layouts\(the default value)]
Changing this to point to msim*.dll without any digits in it (which should still be on your phone even though it isn't being used) will change the hardware key codes back to normal.
!!! THIS WILL BREAK T9 or any advanced correction features !!!
This is most likely why turning off t9 lends to faster text input, but not as fast as this fix.
Props to dilta over at xda for the utility!
note: this hasn't been tested on foreign roms, use with caution.