In recent weeks, login times on my Ubuntu server have started timing out; both through SSH and the local command line console. Examination of the /var/auth.log yields nothing interesting.

How can I diagnose long log in times on my Ubuntu server?

I should mention, also, that no updates have been performed since the problem has started, and that the /, /boot/ and /usr/ file systems are mounted as readonly.

[Edit] This is a stand alone machine, so it doesn't authenticate with Active Directory, LDAP etc. Also, the login prompt is responsive, as is the password prompt. Upon typing the password then CR, I'll timeout. After four a five tries, I will be able to login, although I'm worried this will start taking longer.