[wishfish]

On my M1 MBA, it's sluggish but still usable. Definitely worse than on Windows. It's probably the worst out of the few Rosetta apps I have. It's a little funny. The Steam client performs worse than any of my Mac games that are also using Rosetta. A simple client falls over more than a game which is (visually at least) doing much more complicated things.

[neurostimulant]

IIRC Steam client uses Chromium Embedded Framework. I imagine it'll be as slow as other Chromium (or Electron) apps running under Rosetta.