GL-AR750 losing wifi speed after adding port forward

hi guys,

I am having this issue now after I added port forward and NAT source on LuCI, I was able to resolve one issue of NAT on Xbox, but after I did that now I am losing around 10mb/s download speed even if I disable those rules. If I connect directly to ISP I get full speed, but once I connect my gl-ar750 router and do a test every time I lose wifi speed. what should I do? I need to use port forward and NAT source to get NAT open on my Xbox but I do not want to lose wifi speed, any help will be appreciated.

Can you share your settings?