WireGuard MTU effects SpeedTest

I have GL-MT300N-V2 Mango with v4.3.11 firmware, running WireGuard server.
I use my mobile phone as a Client in 4G network. The problem that I noticed is with MTU value and related to SpeedTest app.

  1. With the default MTU ‘1420’ the SpeetTest app doesn’t perform the test at all, but the internet in the browser works.
  2. With MTU ‘Auto’ the download speed is 30 Mbps.
  3. With MTU ‘1024’ the download speed is 61 Mbps, twice the ‘Auto’ MTU.
    I tried multiple times. The result is always the same. See screenshots below.

Can someone explain this weird performance?
Is it possible to change the default MTU value for WireGuard Server, so for every new Client config it will be 1024, instead of 1420.

If your WAN MTU needs to be smaller than 1420 to prevent packet fragmentation, and you have it set to 1420, you are causing some unneeded overhead by fragmenting packets, thus slowing down transfers. You need to determine what the optimum MTU is for your configuration (google this as there are many resources to help with this). You can set that in each client configuration, but it is generally not needed on the server but you should use the same test to determine if it is needed. As for setting it for all clients, this may not be needed depending on the connection of each client. You can set it low, like you did to test, and it will mostly work. It might cause extra overhead but increasing the number of packets, but not nearly as much as fragmenting too large of packets does. 1300 seems to be a common number I see for folks to use with wireguard clients struggling with MTU issues.To set this, look at the wireguard client config on the client device and adjust the MTU there.