If you pay attention when it loads, you'll notice the Valheim World Generator (the website at https://valheim-map.world ) runs on the Unity engine... just like Valheim does. This is pretty much required for the RNG to work out the same, thus allowing the site to actually follow the same proc-gen algorithm and show you the map that the actual game will generate, using the seed you input.

You're essentially running two games at once; of course it taxes your system a bit.