I tried Windows ToGo on a few USB keys (including two high-speed ones), never managed to get something I could actually use that was not laggy AF, to the point it’s not usable (dozens of minutes to boot, lags of entire minutes and so on). Did I do something wrong?

You can update Tumbleweed once a week, or even once a month without problem. I think the added value of Slowroll is rather slower, hopefully even more consolidated QA no?

It may feels that way to you, but KDE, and especially Plasma (since Plasma 5) has been designed by professional designers. We owe this notably to Jens Reuterberg who created the Visual Design Group within KDE, a group that is still very much alive. The feeling probably rather stems from the fact that KDE’s vision for design is less inclined toward a strongly polished, opinionated interface, but rather to preserve user’s choice?