Is PathAuto module a memory hog?
It depends greatly on which version of Core you are using. The main memory issue is with Token module for Drupal 6, when given a node will generate all possible tokens for that node, regardless of the tokens that will actually be replaced. This means that expensive tokens that need to load or render other objects, etc, in addition with the sheer amount of tokens generated causes the issues.

With Drupal 7, the improved token API only generates tokens that you actually use and greatly improves performance and memory usage. In normal use, Pathauto really only matters when an item is being saved but does not affect "normal" page views.