When magnification is >= 75%, HQ (Fast) noise reduction is indeed used in the Preview window, independently of whether PRIME or HQ (Fast) is selected.
When magnification is < 75% or while a slider is moved, we apply a different raw conversion (including a simpler noise reduction algorithm) to speed up the display. This aims at delivering results similar to HQ (Fast), but it’s not strictly equal.
I strongly recommend to visualize the image at >= 75% to have meaningful feedback on denoising sliders when using HQ (Fast). And you must use the magnifier in the Noise Reduction palette to have meaningful feedback on denoising sliders when using PRIME.
About the GPU, I can give some insight on what we do today. But please note that these details are subject to change at any moment, as our processing evolves. Currently, the Mac version of PhotoLab 2 does not use the GPU much. The Windows version makes better use of the GPU, but mainly during image export (and it does not accelerate PRIME). So, as of today, to get fast slider updates, the best thing you can do is get a fast CPU.