Hey @AnonymousAnt,
It is expected that a relatively large hit to performance will be seen when running a VPN server on a router, and it is generally considered that you will not be able to see your full connection through a VPN. When it comes to Wireguard, the encryption and decryption processes come with a relatively high cost and can bring your speeds down to these levels.
As a general comparison, using OpenVPN on the AX55 could often produce 10mbps connections. While Wireguard is much more efficient than this, it still suffers the same fate and will slow your connection unless you have the hardware (think Business-Class Products) to support higher bandwidths. (We even have a specific VPN Router on our Business Side)
Keep in mind that the VPN Server function is primarily used to access your network resources while away from home, such as cameras or media servers, and is not necessarily intended to funnel all your traffic, especially while connected to the same network running the VPN.