Mullvad + Wireguard on Qubes-OS

Can you try reducing your MTU to 1320 or 1280 in the app?
Also, apply this iptables rule in the ProxyVM:

sudo iptables -t nat -I POSTROUTING 3 -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu