WireGuard Causing Slow Internet on Marble (GL-B3000) Router - Need Help!

Recently, I encountered an issue where, after enabling WireGuard in B3000, I noticed that my overall internet speed became extremely slow, dropping to around 1-2 Mbps. I actually have a 500 Mbps internet connection, so this is definitely not normal. Do you know what could be causing this? After I disable WireGuard, the internet speed returns to normal. However, enabling WireGuard shouldn't affect my overall internet speed, especially since I wasn't even connected to my Wireguard. Are there any configurations I should check?

Did you enable WireGuard Server or Client?
How does your network look like?

I enabled Wireguard on the B3000 as my server and the MT3000 as my client. The internet speed was normal for the first 2-3 days, but it suddenly slowed down. I have two LAN Ethernet connections with the B3000, and they are also starting to slow down, along with the Wi-Fi.

However, once I disable Wireguard, the internet speed returns to normal.

550MB when disable WireGuard
250MB when enable WireGuard.
happens to all my LAN ethernet (with / without connecting with MT3000)