It's a bit heavy on cpu/gpu, it can get hot and low end PCs can get low FPS
More info on the second pinned post:

Valheim running CPU/GPU hot
Even though Valheim has lower polygon graphics, it uses raycasting, physical calculations, point light shadows, post processing, particle effects and many other things that other modern games also use. All of these combine into what may make you experience low FPS in a game like Valheim. In the game settings, there is a setting for Vsync. Enabling this limits the FPS to your monitors’ Hertz; this is something we would recommend high-end computers to use so the game doesn't try to produce the maximum number of FPS it can produce just for the sake of a lot of FPS, which in the end just makes your hardware run very warm when it doesn’t really need to.