Any VPN is CPU intensive, the higher your subscribed WAN speed, the higher the demands. Yes, OpenVPN is particularly CPU intensive due to its design (copying data back- and forth between user space and kernel takes its toll here, while OpenVPN upstream is working on kernel support, it's not quite there for prime time, yet). IPsec and wireguard with their in-kernel design do have a significant advantage here (wireguard, due to the selected ciphers, even more so than IPsec), but they still require a fast router CPU towards the higher end of the WAN speed spectrum.
It is certainly not able to route 1 GBit/s (more like 300 MBit/s, give or take), before even thinking about running a VPN on top.