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I've seldom, if ever, seen documentation on most routers' backplane performance. Generally, especially on small routers, the typical bottleneck is CPU performance. I.e. you'll run out of forwarding performance before you would run out of backplane capacity. The second bottle neck I've noticed is bandwidth of most small router's module slots, especially the WIC size/type modules. This is occasionally noted on Q&A datasheets, and generally only becomes important when working with Ethernet interfaces. Again, though, you'll likely to run out of CPU performance before module slot bandwidth capacity becomes an issue especially if you're also using built-in Ethernet interfaces.
If I were to guess, I would suspect most small Cisco routers use some variant of a PCI bus for their backplane. (I believe recall some of the 7200s actually mention using PCI buses.)
