As someone who previously led development of a commercial VPN system, I assure you, there are about 100 ways for a VPN to go slower than the network hosting it.

As someone who previously led development of a commercial VPN system, I assure you, there are about 100 ways for a VPN to go slower than the network hosting it. Unfortunately. Two cases I can think of are MTU misconfigurations and constrained CPU on either endpoint, where the node CPU can handle non-VPN network demands but can't handle the VPN demand.