MacBook Air has suddenly become very slow at booting and shutdown
Issue: After entering the password the computer would appear to freeze and become unresponsive. After about 20 mins it would eventually log in. User was claiming that browsing the internet was very slow and he could only access one page at a time.

Resolution: I had unbound the mac from the Active Directory domain and it fixed the issue. As it turns out the computer name had been removed from the AD Computers list.