EVERY visa application gets second-person checked - it is looked over for errors, and to guard against the possibility of corruption.
There is no "normal" timing, but it's any queue for the 2pc's attention that will take up most of the waiting, and the actual check is usually very straightforward.
You can only tell your employer what INZ have told you. This is a very ordinary situation, and the accredited employer will know that you, and he, can only wait until INZ have done the necessary. Both of you would be breaking the law if you started before you get your visa. He has said when he would LIKE to have you start, and INZ are doing their best, but having you legally covered by a visa takes precedence over his and your convenience.