Oh, what a tangled web we weave, when first we practice to deceive
I can’t imagine changing WG’s MTU is going to help anything – more likely hurt.
Assuming the ISP is using 1500 MTU, likely if cable/fiber (and lower if PPPoE/etc., and much lower if LTE). And LAN is almost certainly using 1500 MTU. You are going to EITHER get fragmentation or a “constrained” MTU on both sides of a bridged LAN. Regardless of if VXLAN or EoIP. e.g. one LAN packet will quickly turn into many packets over the WAN, especially for TCP traffic.
But changing the WG MTU the upwards is likely a very bad idea. WG is the outer tunnel, so you’d get multiple levels of fragmentation with EoIP/VXLAN also getting fragmented if using defaults. If the WAN’s WG path isn’t 1500 at any point, even the default 1420 for WG may even be too high. For LTE, it be 1348 to avoid WG getting fragmented inside LTE network (assuming LTE is 1428 MTU, YMMV) too.
In theory, the VXLAN MTU should be the WG MTU - 50 bytes for its headers. Now here this is a tricker decision (WG is easy, don’t over-drive MTU – set it to match WAN - 80 bytes!). But for the VXLAN MTU, a lower MTU below 1500 is going to cause the entire LAN to use a lower MTU, since VXLAN (or EoIP) is bridged. And a bridge uses lowest MTU of its ports. But it’s here where you faced with Hobson’s choice Since it’s the inner tunnel fragmentation of bridged packets may be less of a problem, than the problems caused by a lower MTU on the entire bridge. So for testing, I’d leaving the default 1500 as the starting point – still get fragmentation but only on the inner traffic and the MTU difference is masked on either end of the bridge.
If you’re wondering why I’m harping on the subtle MTU and fragmentation part here, check out this thread:
http://forum.mikrotik.com/t/mtu-oddness-with-wireguard-and-maybe-vlans/163247/1
At the end of the day, fragmentation is a sure-fire way to get a lower speedtest. If you like seeing higher ones, don’t make MTU bigger, make it smaller (well, rather, match the upstream network’s actual MTU). And a wrong MTU will very likely break some protocols.