Drupal commons running very slow, what are the key modules to disable?
This is normal for a Drupal site with a lot of modules. There are many SQL queries and huge use of memory to provide these admin pages, but if you look the site as a normal visitor, will see that this isn`t the same.

Commons, specifically, i had many issues related to very slow admin pages, when developing a large social network, with the many modules from Commons itself and many others specific from the project.

To improve this, you can:
 * tune your MySQL (the first thing to do is set your tables as INNODB, that you help a lot)
 * use memcached
 * use PHP APC
 * Look around http://web.archive.org/web/20110516021804/http://www.morningtime.com/Drupal-6x-Performance-Guide/513 (the MorningTime site removed the link, so you`ll only be able to see in the webarchive)