Steam deck CPU sustained frequency is 2.4GHZ, what did you expect? It's also a Zen 2 core which has very poor single core performance. Valheim is very notorious for its CPU bottlenecking when you get complicated builds.

I like the Steam Deck and all, but I'm not in it until they have a (True) Zen 3 or higher core part, ideally an X3D but that might not be possible in the power envelope.