System slow on new account creation
Do you have the CiviMember Roles Sync module enabled? The code in this is very similar to the other 'role sync' modules and is currently quite slow. There is an open issue about it here: https://issues.civicrm.org/jira/browse/CRM-14770

Otherwise you could try putting markers in your code at various points to see where the slow-down is happening. Or use a PHP profiling tool.