Originally Posted by
Ditto
No fast track for J, you could look into the VIP/meet&greet services.
SA immigration are one of the most laid-back ones I've seen (I was once asked to say who I prefer between 2 soccer teams in my home country...), it's just that they are not very efficient, you will see many desks are closed etc.
I booked one for my wife and myself last week and our wait at immigration was less than 5 minutes. Really painless. If the OP cannot adjust his 90 minute connection I would agree with using one of the VIP meet&greet services.