If you encounter a problem where it takes a long time to dial any extension number this could be caused by a few reasons. The most likely cause is that the interdigit timeout is too long. You will most visibly see the effects on a dialplan like the following:

6,1,something
6,n,something
600,1,something
600,n,something

In this case, dialing just a six will take 5 seconds to connect to extension '6'.

This timeout is set in extension.conf where TIMEOUT(digit)=5 is specified in the dialplan. You can lower this number. The consequence of shortening this too short is that users will eventually complain they can not enter a full number quick enough without timing out. Better yet is to try to make the extensions in your dialplan deterministic lengths.