Login as admin takes more than 45 secs

Hi,

I upgrated my Moodle sites from 2.1 to 2.5 version 1 month ago. From this time login as admin takes more than 45 secs, sometimes more than 100 secs. But after login the webpages load quickly (except Home page).  

Login as student takes less than 2 secs. 

Have you got any suggestions?

Thank you.
Ivan Kreskoczi
Slovakia