You need to first know what's NAT (Network Address Translation).
https://en.wikipedia.org/wiki/Network_address_translation

As we know, a CPU of a home router is obviously not as powerful as a computer's.
Thus it couldn't handle too much NAT throughput, especially for some high-end products which required Gigabits speed.
To solve this issue, NAT boosting is added, which is a module that help handle the NAT throughput via hardware or software aspects.
With NAT boost, the networking speed could usually reach up to 1000Mbps.

NAT boost is always switched on by default. However, if manually disable it, then internet speed will decrease to a lower value due to the ability of the CPU.