What I would do is this...
Step 1: 1035 PLZ JNB 1230
Step 2: take the complimentary shuttle to the Emperor's Palace Hotel and Casino
Step 3: find the Tribes steakhouse
Step 4: eat a tasty, cheap steak
Step 5: take the complimentary shuttle back from the Emperor's Palace Hotel and Casino to OR Tambo
Step 6: head back through security and enter the lounge.
Then you will perhaps have three hours in the lounge rather than six.
Caution: while Tribes is good, other restaurants at the Emperor's Palace Hotel and Casino should be treated with caution.
Alternatively, the Nando's land-side at OR Tambo is the best Nando's I have ever eaten at!