Usually optimization is the last step, after you're sure that everything you are optimizing will be in the final application. So don't look for it until Valheim is feature-complete.

Some optimizations are just standard, and programmers use them all the time as they write. What we are talking about here is optimizations that are peculiar to the application, and you can't be sure what those are until all of the peculiarities of the application are nailed-down. With peculiar nails.

Valheim EA may include debugging code and that is a good thing, but not for speed. The good news is that it can be turned on and off with a compiler switch. Which we don't have. Whether it is turned on or not in a given update may depend on whether they are in pursuit of a rare bug. (IDK, but that doesn't stop me from guessing.)

Some things are down to the speed of Unity.