To go back to the reason, set calls windows fun too that notifies programs of settings change. The slowness/fastness is by design and only Microsoft can change it.
If you want to change something faster, write the variables directly to registry.
I find that hard to believe, but it's MS, so I am not sure why I find it hard to believe. A CLEAN BUILD Windows 10 takes 2 seconds per set command, yet an Upgrade Windows 10 (from Windows 8 pro) takes 6 seconds per SET (before the upgrade, it took 0.1 seconds to run), yet, Windows 7 Pro (clean build) takes 0.1 seconds. If it was MS, then I can assume that all systems would behave similarly, yet they don't. Hopefully a solution will show up...
