The update manager module makes admin pages very slow, what might be the causes and how could I solve this problem?
Update module is slow when site is behind a router or firewall:

https://drupal.org/node/369145


  The Update Status module attempts to get update information using the
  internet. Many servers have firewalls that block this.


That means that Drupal tries to access the Internet, but if the site is behind a router or a proxy the petition probably is ignored, so Drupal is 'stalled' waiting for response. When Drupal is tired of waiting (the petition timeouts), usually several seconds after, the page request continues and finllay Drupal delivers the page. You feel that the site is slow, but in fact Drupal has been waiting same as you.

The solution is to disable Update module in dev environments, or allow the site to connect to Internet to get the update information.