Tunnels have overhead. MTU overhead itself can cut 5% of your total bandwidth as a default (1500 -> 1420). Forget all the side-channel control stuff.

MTU itself is an interesting issue for wireguard. It defaults to 1420, which should be fine in most cases as the default is 1500 for most ISP connections. But there are interesting cases where you need to go less… If you try to cram a 1420 MTU packet down a 1440 MTU ISP connection (you need 28Bytes overhead minimum, so would need 1412 in Wireguard in this case)… you’re rewriting a fuckton of packets and splitting tons of data that can ruin your connection speed (halving immediately).

I have seen some people recommend 1384 MTU before… The lower you tune this for compatibility the less speed you get.

Once again though… this is way over a normal users head. And likely even over yours since you don’t seem to recognize that this is happening and that it isn’t byte per byte the same.

You should expect wireguard to lose you 5% speed minimum… with other issues potentially making it worse.

Edit: clarification on a sentence cause the wording was bad.