bug#3978: 23.1; Slow scrolling

In the new version I have been experiencing slow scrolling, more
noticeable when I use a small bitmap font that can display 100x200
characters, or even 6x13.
It's slow enough that if I hold C-v it won't redisplay at all, until I stop.
Also, a package that uses overlays for displaying a fake menu
(company-mode) is slow.

My platform is Windows XP...

Thanks