So the 80mbps on your Pi 4 isn’t a limitation of the CPU (it’s purely the internal wireless)
With the USB WiFi Adapter and cabled WAN connection to router you can get up to ~600Mbps for the max speed over WiFi (if you get speeds of 500mbps normally then could likely be 400-450mbps max with VPN)
VPN speeds as always are slower than unencrypted, but WireGuard especially runs fast on Pi