There are a few things which need to be pointed out on this forum which have either been ignored, or incorrectly posted.
CDRW - yes there is an indication on the PFD as to how much input the PF has used. This disappears shortly after take off.
Felix wrote about the altitude and temperature at JNB. Anyone remember the WAT limit, or Weight Altitude Temperature limit for maximum weight at take off? You can bet that the weight will probably been performance limited, so any overload, even 3 tonnes will make a serious dent in your performance, and would have been a contributory factor if true.
Most of us are awaiting the outcome of any enquiry but there will of course be supposition and conjecture at the moment.