Still slow as **** and a generally deeply unpleasant experience which doesn't even come close to Spotify's Mac app, which is itself pretty bad. It's no worse on Big Sur than before, but I can't say it's any better either.
It's got a long way to go before I can wholeheartedly recommend it. Only reason I stick with it is that it allows me to change the song metadata in my library and relatively straightforwardly upload MP3 files to my library to listen to on my phone.