Strange behavior (SDL-0.10.0)

First off Rafael, thanks SDL_MapRGB does what I need.

Now to the strange behavior…
I’m cleaning up a my library functions(1) and I noticed that the frame
rate I get can vary based upon the location of the window on my
desktop. For example, a 640x480-16bpp SDL-window (X is running at 16bpp
also) in the upper right corner of the screen will easily get 30 fps in
my test, but put it in the lower left and it struggles to get 24 fps.
Is this a known issue?

(1) Basically a bunch of functions I wrote so I don’t have to re-write
the same SDL code for every application I make.