Just wanted to make a quick share on how to solve one weird Drupal 7 performance related problem.
Picture this. Yesterday, your Drupal 7 site run pretty ok, but today it as slow as a snail and you don't remember yourself changing anything. I myself encountered this problem today and spend a good hour looking for a problem source.
My problem was connected to a so-called "Avoid re-scanning module directory when multiple modules are missing" bug which is still not resolved. This problem happens if you have some missing modules in your Drupal installation. For example, you could delete or rename module while it's enabled (which happens often on a development machine).