In the last few days, my SSH connection from home to office is very very slow. However, when it's connected, the speed is not so bad. After some -vvv debug, it seems the SSH client waste a lot of time before showing a line "Cannot resolve network address for KDC in requested realm". What? SSH is using Kerberos? That's bad.
Well I have done some Kerberos programming jobs recently on this computer, but I never meant to tell SSH to use it. Finally I add these 2 lines into ~/.ssh/config, and now it's much faster.