Admin pages including content edit are exremely slow I am using latest 7/16 version of core and complicated field collections with all kind of stuff. The problem is it takes up to 1 minute to load all kind of pages in admin section. All dev modules and update manager are disibled.

How could i know what exactly cause such a low performance? What can i do to improve it?
best answer: You need to profile the PHP code in order to see what is causing the slowdown. Also check firebug to see if anything is taking a long time to download. Go through all the slides  from my DIY Drupal 7 Performance presentation.

quick answer: You can try the first 4 patches listed in the Performance wiki as these are things that I identified on our setup as being slow.