The lower speed seems to affect all Iris upscales to anything higher than FullHD. So upscaling SD to 4k or 8k is also considerably slower in the new alpha than it was before.
Still, my most used case is SD->FHD and there this new alpha shines.
But integrating both codes from the new and the old version and chose the faster one based on the output resolution would be an idea. Unless you find a bug that affects rendering speed with the higher resolutions in the new code and are able to speed this up as well, of course 
