If your project is 32b float, set Warp Mode to “Check UVs” – do you see white dots?
If so there migth an issue with your UV map generation.
Need more info here (format, render generating it in what app…)
As for speed, what size is the UV image (I know you said the texture is 4K)?
Yes it would be nice to have some GPU magic here, as you have seen we just gor RE:Flex on GPU which has some parent code. So the hard work is being done in background, not sure when this will happen still though.
Pierre