The throughput the router can deliver, from external side to internal computers (and vice-versa), is a total of 80mbit/sec. Or less if more stuff in the router is enabled.

The reason they quote "NAT" thoughput, is that "NAT" requires software (CPU) involvement on the router for each and every packet passing from internal to external and vice-versa, so communication speed is dependent upon the speed of the processor inside the router.

LAN speed on the router, say from one PC to another, is unaffected, as the packets there are switched directly by hardware, at full wire speed (100mbit/sec). No software involved.

"NAT" (Network Address Translation) is the core feature of most firewalls, enabling many internal PCs to share a single (or a few) external IP address. By itself, it is often adequate. Most firewalls also do "SPI" (Stateful Packet Inspection), which simply means the embedded Linux kernel is trying to block a lot of protocol violations in addition to doing NAT. And SPI can get a lot fancier, depending upon the features enabled.

But using SPI takes more CPU horsepower inside the router, and will probably lower the maximum NAT throughput.

The only way to really know if it's "good enough", is to try it and measure it.

Cheers