If anyone else is having performance issues, I followed the advice in
this video and it's like playing a different game. If you don't want to watch a video:
If you don't have a Valheim desktop shortcut, you can create one. Then right-click it, select Properties, put your full game path in quotes (it should already be filled in, just needs the quotes). Then, before the quotes, add C:\Windows\System32\cmd.exe /c start
So, my full path looks like C:\Windows\System32\cmd.exe /c start "Valheim" /high "D:\Steam\steamapps\common\Valheim\valheim.exe"
You can skip all those steps, you would just need to open Task Manager, right click Valheim, and select High Priority every time you run the game.
Then, in game settings, set particles to Medium. No visual difference but gets a performance boost.
And finally, go to Steam > Steam Apps > Common > Valheim > valheim_data and open the boot.config file with Notepad. Paste the following lines at the top of the file:
gfx-enable-gfx-jobs=1
gfx-enable-native-gfx-jobs=1
You can also right-click Valheim in your Steam Library and add the following to the Launch Options: -window-mode exclusive -molluc=system -USEALLAVAILABLECORES
It also helps running in Fullscreen mode, rather than borderless fullscreen.