The limiting factor, for the software based routers, is the performance of the CPU. As the traffic load increases, so does the CPU load. When the CPU load approaches 100%, router is unable to continue to increase amount of traffic it can forward.
Beside just forwarding traffic which consumes CPU, additional configuration features will increase the load on the CPU for the same amount of overall traffic. Features such as ACLs or QoS are examples.
Software routers have an optimized path that processes most "normal" traffic. Some traffic, alone, or depending on the features being used by the router, will cause the router to process traffic in a much less optimal manner which is often about 10x or even more slower. This is known as processed switched traffic.
The WAN bandwidths you've seen listed are very conservative, but they do tend to insure you're unlikely to encounter a performance limitation if you go by them.
Some make the mistake when they see a small router with a FastEthernet or GigEthernet interface, that it's capable of routing LAN traffic at such speeds, but that's seldom true. For LAN routing, the L3 switches offer much, much more performance although usually with a loss of some of the more advanced WAN routing features. (E.g. LAN L3 switches QoS is often limited compared to small WAN routers.)
As a comparison, the 3845, the most powerful ISR is rated at 500 Kpps (good for about 335 Mbps Ethernet for 64 byte packets) but the little 8 port L3 switch, the Catalyst 3560-8PC, is rated at 2.7 Mpps (good for about 1.8 Gbps Ethernet for 64 byte packets).
