unlike what martin said, it has nothing to do with your drivers.

the game is just unoptimized. the majority of the time it doesnt even have anything to do with your GPU. valheim is CPU heavy. and it doesnt help that it basically bottlenecks itself because it doesnt utilize multicores very well. pretty much anything with more than 4 cores is a waste.

this has been one of the biggest complaints since the games release. seriously theres at least a post a day about it on steam or reddit, and its always followed by random false reasons that people just make up.

bottom line, the game is not optimized. there have been several posts about it on reddit where folks have gone deep into the issues of the game code.