I’ve got just a few lines of AGAL that I add to the BasicDiffuse method and I am getting some very bad performance.

The AGAL code uses the global-position to determine its color. When even chaning a small amount of output pixels it goes down over 30 FPS. It’s crazy.

Are there any AGAL pitfalls that I should be aware of?

The severity of the drop may be related to your refresh rate. If your backbuffer swaps are vsynced, with a 60hz refresh rate, a slight performance hit that would sink below 60fps will have to wait until the next refresh cycle. That means it can only draw 1 frame per two cycles, resulting in only 30fps, despite only a small real performance hit.

Sounds like a possibility?