[metamatt]

He's saying if he could optimize away all the requests by crawlers and robots, he'd have half as much work to do. Doing twice as much work costs more -- on the server side to serve it up, and on the network side to transfer it.

It's not literally a speed comparison, but you can imagine how if you could optimize away all the robot requests, you could do something else with the time and resources you'd formerly spent serving them.

The smugmug addendum to the story explains this pretty clearly.