Don't know about Spirit Ops specifically, but I took new deliveries of CRJ-700's from Bombardier at another airline and after importing the aircraft from Canada, the aircraft have to go thru a conformity check with the FAA, and get approved to get placed on the airline OpSpec as an authorized aircraft and the aircraft has to get accepted into the airline's weight and balance program and maintenance program, prior to being released to the line.

Prior to that, the airline's normal weight and balance and maintenance procedures don't apply, and at least with the CRJ-700's it was back to weight x arm = moment for everything that's on the airplane - unusable engine oil, water, ballast, each passenger, catering, catering carts, blue juice, etc. just like private pilot days, convert that to %CG MAC and then figure out takeoff trim setting.

All of the aircraft parts had to be entered by serial number into the maintenance computers for tracking for whatever purpose. And if something broke prior to getting introduced into the airline maintenance program, our airline mechanics couldn't work on the aircraft.

I think with Spirit, they have to get authorization for the specific aircraft to operate in another countries' airspace by N-number. There is a dispatch document which specifies where each aircraft is not allowed to go until they get authorization from the foreign country.

Just my guess anyways as to why it takes so long to get an aircraft online. It was like a 2 day process for the last airline.