Some of these low end home NAT devices labeled as routers aren't the biggest beasts out there. Trying to start hundreds of connections, and receiving a number of connections is going to clog the machine a bit. The CPUs are teeny weeny, and the ram is barely measured in megabytes (by today's standards 😉). Obviously this keeps power usage and heat production low. A tiny, quiet, and relatively cool (these things get hot, IMO) device that is quickly plug and play appeals to a lot of people. It did when I NEEDED to get a router ASAP.
They're typical consumer products: underpowered for even the averagest of geeks, but perfect for email, IM, and web browsing.
Internet connections can also become quite bogged down. Uploading in excess can often stifle downloads, and royally kills web browsing (which is why my brother's computer got unplugged from the internet daily :evil: ).