Yeah, I played through the whole thing on my steam deck and it was mostly a great experience.
The last act was, performance wise, pretty rough though. Maybe there was some setting I could have changed to make it smoother, but the framerate was only about half what I got during the other acts.
Not a raspi, but I had similar issues on my opensuse HTPC which turned it to be related to issues with (or missing) media codecs in Firefox.
After (re)installing all of them, it worked like a charm.