Originally posted by Steve:

[...]
Obviously you do this at your own risk, so back up your save data.

https://jamesachambers.com/revisiting-fixing-valheim-lag-modifying-send-receive-limits/

Edit: Oh, not sure if this makes a difference or not, but I also change the priority for the server run at "above normal" (set in Windows Task Manager)

Can confirm / it works.
Sure it doesn't solve the core problem, but it helped our group a lot.
For the core problem the devs have to rewrite the code from the scratch, because they use an p2p-driven server model in their code.

Read the page exactly and all the comments there to understand the problem correctly before changing anything.

With everything the devs add, more data needs to be send and the problem will get worse every update if the code doesn't change. Sadly.