[wakeywakeywakey]

You probably just need to lower your MTU if your phone is getting an ip6 address.

[viceconsole]

Even with the minimum of 1280 for IPv6, nothing improved.

[josh3736]

The much more likely culprit is your VPN server's port. If it's running on some no-name port (such as the default 51820), that's likely to get throttled.

I'd bet that switching your VPN server port to 443 would solve the problem, since HTTP/3 runs on 443/udp.