Connectivity Issues - PPPoE WAN, WireGuard and Mullvad VPN

[foss-johnny]

I've configured Mullvad via Wireguard and am using a PPPoE WAN connection.

I have it working, but find that when a client is connecting to various internet webpages, it takes more time to load, and some of the content or functionality does not work.

E.g. Speedtest.net doesn't find the optimal server, and is extremely slow to start the test.

I've installed wireshark on my client computer and can see a fair amount of TCP retransmission messages in the traffic.

I get the sense that this is related to MTU/MSS, but I've changed it to different settings and can't seem to get it working properly.

My WireGuard instance MTU is 1412, the WAN PPPoE MTU is default (1500) but I think opnsense reduces it automatically to 1492, I have Normalization rule set to Max Mss 1352.

Would appreciate some help to determine what's causing this?