JNB Departure time
Look at the current timings
Depart HKG 2345 arrive JNB 0625 / depart JNB 1230 arrive HKG 0715.
If you bring the JNB departure time forward 2 hours then the 40% of passengers who connect from Cape Town and Botswana can't make it the same day and may require an overnight - you will loose business. Also the arrival time HKG would then be 0425 - passengers would love that. You would have to change all the arrival/departure slots not just the JNB departure slot and slots are like hen's teeth now.
If you change the departure until early evening (say 4 hours later) then the HKG arrival is the middle of the day HKG and a lot of the connecting flights have already departed and the passenger has to wait until the evening for the next round of connections.