Optimization

To optimize UDP traffic

If you have a router with enough CPU power, in System->OpenMPTCProuter, wizard tab, "Advanced settings" checkbox you can choose V2Ray instead of Shadowsocks as proxy. V2Ray can transport TCP & UDP, Shadowsocks transport only TCP so UDP is transported via the VPN.

Else you can try another VPN, like Glorytun UDP.

To optimize TCP traffic

In Network->MPTCP, you can try another TCP Congestion Control. BBR2 seems to offer a good result in many case (available only on 64bits arch), BBR or BBR2 give a good result only when connection are good without packets lost. Cubic work in every case but can be slower.

If you have a bad connection, you can increase TCP Syn retries.