The point he's missing is that you don't have to develop software on the same computer you test it on. The best solution is to have a separate test computer that's appropriately slow. In fact, most people probably already have an old one lying around that they could use.
And regardless of speed, it's good to test your software on a variety of systems.
I certainly do test on many different systems, but not all of them.
This doesn't take the place of real-world testing. It takes the place of setting up a network and putting a modem between my client and server then attaching a tape drive to my server to see what happens.
So far, it's been quite useful, but I've got a ways to go.
And regardless of speed, it's good to test your software on a variety of systems.