We have written before about Drupal Memory usage by modules, and the Open Buffet binge syndrome.
But this time, it was different. Modules were not to blame.
While inspecting a site that had several performance problems for a client, we noticed is that memory usage was very high. From the "top" command, the RES (resident set) field was 159 MB, far more than what it should be.
We narrowed down the problem to a view that is in a block that is visible on most pages of the site.