i personally seen a performance increase by making steam force true full screen mode.
right click valheim and go to properties and add this to addition command line arguments.
-window-mode exclusive -screen-fullscreen
additionally i would make sure to go into your gpu software and optimize your gpu specifically for valheim. I personally play i two different pcs one of which requires additional optimization for games where as my main pc doesnt.
Make sure your gpu software profile for valheim has everything to highest possible performance.
Hope this helps anyone see better performance and higher fps