I play the native version and mouse speed seems ok to me. Maybe you can try to set it manualy in ~/.config/unity3d/IronGate/Valheim/prefs where i have <pref name="MouseSensitivity" type="float">3</pref> which seems to be max one can set in the game settings.

Edit: with Proton the path will be somewhere where you have Valheims wine prefix ofc, sry:]