Dropping packets when openwrt is forwarding openvpn udp traffic

Hi, when I used a UDP VPN on my PC that is connected to a openwrt router, the router slows down and starts dropping packets. The VPN in question is a surfshark vpn in udp mode.

The slowdowns start when the VPN starts forwarding a couple of packets. For example, doing a speedtest while connected to the VPN makes the router slow down to the point of dropping lots of packets for the whole network.

When I configure the VPN to use the TCP protocol everything is fine. The router handles it perfectly.
Software flow offloading is enabled on the router. But disabling it didn't fix the issue.

Kernel Version: 4.14.167
Firmware Version: OpenWrt 19.07.1 r10911-c155900f66 / LuCI openwrt-19.07 branch git-21.189.23240-7b931da