@take3 the video is not lying, look at the timeline : it’s only a 10 seconds sample processed in Extreme mode - it should actually take less than a minute to unmix those 10 seconds actually.
That being said, I can tell you that full GPU support for Unmix Song is now working with the upcoming patch 
It now takes 53 sec to unmix a 3min song in Extreme mode (without Sax & Brass) on my RTX 3090, or 16 sec in High mode.
With Sax & Brass included, it takes 73 sec in Extreme mode, and 20 sec in High Mode.