Why is it hard to believe? You are searching billions of web pages. A typical index containing millions of web pages takes up gigabytes of space and fits in RAM

Why is it hard to believe? You are searching billions of web pages. A typical index containing millions of web pages takes up gigabytes of space and fits in RAM on a commodity server. In order to run one search really fast over billions of pages, you need an instance of a distributed index that is three orders of magnitude larger than what fits into one machine.