We were recently troubleshooting a client site running Drupal 7.x, and main complaint was high memory usage on all the site's pages.
We were able to diagnose and solve two main causes that range from the common to unusual.
This is a Drupal 7 Commerce site with 173 modules, and 3 themes enabled. Apache Solr is used for search, and there is custom code to talk over the network to a non-Drupal backend server.
The site runs on a Debian Squeeze Xen VPS.
For most of the site's pages, the client was seeing high memory usage, as follows: