Internet speed slow

I have a problem: after I use a VPN on my router with OpenWRT, my internet speed becomes so much slower. From 50 Mbps to 6-7 Mbps. What can I do?

Run htop, see if your cpu becomes a bottle neck when running the VPN.

I would guess you need either a router with faster CPU(s) or a dedicated crypto-acellerator or a less CPU-demanding VPN-type.