It seems normal that the speed will cut down once NAT boost is disabled.
Because when you disable the NAT boost feature, all the traffic will be handled and forwarded by the chipset. As we know, a chipset on a router is obviously not as powerful as a computer's. Thus it could only handle about 150Mbps when you're setting bandwidth control feature.
It's a normal phenomenon as seems you need to take up more resources of the chipset to control the traffic by reducing the throughput.