Why is OSX with rotated display slow on Mini?

I like to use the rotate display option in "Display preferences" under Tiger (comes in handy to display whole pages while keeping them readable on a 17 inch screen). However, once I turn on this setting, OSX becomes suddenly terribly slow, e.g. when I move a window the movement lags behind the cursor and is very choppy etc.

My question: is this an issue that's specific to the Mac Mini ? Is it a harware or a software problem? [Maybe the graphics card is somehow optimized for landscape mode and can't handle portrait mode?] Is there something one can do to fix this?

Thanks for any hints!

 

I am surprised though that OSX should need CoreImage for this effect. One would think that it merely needs to exchange the x- and y- coordinates of its display coordinate system, which doesn't sound like it should require a lot of processing power.

 

That was an interesting hint - thanks! I did install the ATI utilities and it did indeed offer another way to rotate the screen. Unfortunately it doesn't seem much quicker than the feature built into Tiger.