Low redisplay performance (23 regression)

[David Reitter]

I've been noticing a substantial slowdown in redisplay performance.   
Scrolling down emacs.c in fundamental-mode takes about 2 seconds in  
Emacs 22 (Carbon port), and 3 seconds in Emacs 23 (NS port).

With font-lock-mode (in c-mode) enabled, the difference is even larger:

About 2.5 seconds in Emacs 22, and more than 7 seconds in 23/NS.

The effect interacts with the display of overlays etc. in the header  
line.  When turning that off, I get about 2.5 seconds in Emacs 22, and  
still more than 4 seconds in 23/NS.  (All values estimated manually  
from several tries.)

I'm wondering if others get the same on other platforms comparing 22  
and 23, or if this is a problem specifically in the NS port (for  
example, with setting drawing color or the like).