The Game is pretty CPU bound and doesn't scale well with many cores. You can try the new Vulcan mode or try this in valheim_Data\boot.config This shifts some workload from CPU to GPU.
Be warned that Vulkan AND this options enabled may boost your FPS significantly, but will occasionally crash your game - including resetting you Character to a previous save (it obviously doesn't save if it crashes and your Chars are stored client side).
So try Vulcan or this options, not both.