Non-pilot speaking. On the LHR~JNB route, that I very familiar with, my private log shows that sector times for the VS 340-300s are about 30 mins more than for their 744s.
I have not done the route with other carriers for a while now, so cannot offer comparisons. VS always show that the ex-LHR arrives 30 mins after the 744 and they schedule the JNB departure 30 mins earlier for a Bus. That said, "30 mins" is a nice round figure for folks and it might be +/-.