Are you using Wireguard as a client or server?
Not Sure what your set up is but it could be any number or things.
It sounds like your ISP is throttling. Most ISP will boost downloads for 20 seconds and slow down until bandwidth is available. You 30Mbps is a constant tunnel.
Mbps (Megabits per second). Also your speed is split between the two LAN ports and you will loose some depending on how many devices are infront of the brume to the Gateway. Your DNS might need to be reconfigured also.