I don't think CPU speed per se (how fast a single CPU is) is relevant. It's all about cost, since most IP networks are free to balance the number of CPUs, system buses, network links, etc.
And from what I've seen, as the cost of routing in a general purpose CPU has come down, so has the cost of doing it in a specialized network link processor (what we're calling "hardware" here) -- assuming the IP header structure is simple enough. So today, as ten years ago, people would rather do routing in an ASIC than allocate x86 capacity to it.
I think system designers balance system bus and CPU speed too, so it's not the case that there are lots of idle cycles in the CPU because the system bus can't keep up with it.
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