Mtus are diminished by 80 bytes because the WG interfaces are connected in series. But that's not a problem.
In the simplest setup provided by OpenWRT guide to Wireguard setup, the speed drops as descibed in this post: Wireguard dropping connection speed by almost 80% - #18 by efahl
Maybe someone else with the same router has the same problem and solved it?Or came to a conclusion the problem cannot be solved, maybe.