++EDIT IIRC, Proton's website said that openVPN UDP should be used, or is the default.
BUT, Proton has about a half-a-dozen different installation pages, CLI, Debian, Linux APP, and one specifically for MX, and others, I believe.
But I don't know this stuff.
++EDIT2 I stand corrected:
per: https://protonvpn.com/blog/openvpn-vs-wireguard/Proton VPN now uses WireGuard by default because it is secure while also being faster and more efficient than OpenVPN.
Wireguard built in kernel: https://arstechnica.com/gadgets/2020/03 ... ux-kernel/
Cheers!