I've noticed it too, idk why the game is that way, but I combined some fixes I found in some other places and it worked wonderfully for me.

1. Go into your Valheim install folder, right-click Valheim.exe, and click Create Shortcut.

2. Right-click that shortcut and click Properties. Paste this into your Target line, after filling in your install folder in the given space:

C:\Windows\System32\cmd.exe /c start "Valheim" /high "<VALHEIM INSTALL FOLDER>\valheim.exe" -window-mode exclusive

Ex: C:\Windows\System32\cmd.exe /c start "Valheim" /high "C:\Program Files (x86)\Steam\steamapps\common\Valheim\valheim.exe" -window-mode exclusive

3. Go into your launch options on Steam, and add the following line, inserting your own info as needed:

"<VALHEIM INSTALL FOLDER>\<YOUR VALHEIM LINK NAME>.lnk" %command%

Ex: "C:\Program Files (x86)\Steam\steamapps\common\Valheim\valheimshort.lnk" %command%

This will run the game in high priority automatically, and in exclusive window mode. After doing this, I went from ~6fps on minimum settings in my huge base to ~30fps on maximum settings in the same area. If you want access to the console, you can add the usual "-console" command to the end of your Target line in your shortcut.