Hey Eric,
This was introduced very recently...
I also confirmed it is doing the same thing in 4.0b4 with no other extensions installed...
To reproduce:
1. Open PWM (I always click the ring icon in the Statusbar)
2. Select an account
3. Modify the 'Using Text' field - it updates really slowly, like, one character per second or so
4. SHIFT-TAB to the MPW field - it updates aslowly too, but not quite as slow
Hopefully its an easy fix, cause its driving me crazy when I have to log into one of our users accounts (especially if they have a long username)...