Edit: I was too quick to judge your post. The article is indeed just about serving content, not about why their results are fast.
Here's a handy rule of thumb about relative costs: the cost of a byte on magnetic_disk:flash:ram is about 1:10:100. I.e. the cost increases 10x when you go to the next level.
The article compares the speed to other websites like nike.com which have nothing to do with a search engine.