Pagerank is a funny algorithm, think of it as a problem in gravitation, every page affects the pagerank of every other page.

You compute the equilibrium and then normalize the results.

There are a bunch of optimizations that you can apply in order to speed things up bit it is still quite computationally expensive.

For an overview of PageRank, you should check out http://www.ams.org/featurecolumn/archive/pagerank.html