One day our website’s wp-admin didn’t open after fill username and password then hit the login button. it was loading continuously but not able to see Dashboard but some times it took long time to open Dashboard, We searched many forums and blogs but no solutions, then we decided to open functions.php file of current theme and was looking each line one by one then we found that there was some CURL coding were there, looked that coding it was a functions to update the theme automatically once a day when admin logs into wp-admin, and commented that function and tried to login again wp-admin, Yes, it worked successfully, we got Dashboard.
1. Look into functions.php file of current theme
2. Look into plugins, if some plugin may download external contents or send some data to external server, If external server is slow , then our site will also be slow.