Quick noob question

If I am running a shader pass which renders to a 1x1 pixel texture, that shader pass only runs once per frame, right? (Or 8 times to a 4x2 pixel texture, etc.)

Correct.
But mostly some complex math that has small output data has huge compilation time, so this is more a culprit than execution time.

When you mean compilation time, do you mean like when ReShade starts up, processing effects? In terms of execution time, I can definitely see how crazy math on one pixel might take a lot longer than simple operations on many pixels - especially because of how the GPU parallelizes everything.

Otherwise, that makes perfect sense, thanks!

Yes, at startup. A shader with an unrolled loop takes some time to compile, no matter the amount of pixels it runs on. Calculating avg color of screen by averaging as many pixels as possible is an example for heavy compilation time, even with a 1x1 output so performance hit is almost non existant.

Interesting, very good to know!

Cheers