That's one of the factors. Generally the optimisations that happen first are on network path length / network equipment induced delays, as there's relatively cheap and quick gains to be made. The bigger delays are invariably in your applications that are processing or generating data, which are more costly to optimise.

That said, the Fixnetix stuff is only talking about one aspect of what's involved, and about as representative of reality as Cisco's published WARP speed figures in their Nexus 3500 range.