OS X Mountain Lion Terminal and tabs take ages to load
Did you look at your .profile file (vi ~/.profile) to check if someone (some program) has added something there that takes the time?

Try to create a new user on your Mac (System Preferences -> Users), log in as this user and see if the problem also happens.  If not, compare this user's .profile and the one where the delay happens (or at least the experiment will tell if it is something system specific or user specific).

Also there is the global file /etc/bashrc which is processed for all bash shells for all users.  Something added there could cause a delay for all shells for all users.