I've looked at the differences in xp gain, and for example the first tiger quest in Stranglethorn gave me 1,300 xp, whereas it gave Joana 1,250. This is a very small difference. At higher levels the gap is wider, for example Mission Accomplished in Western Plaguelands (after finishing all four cauldrons) gave me 13,500, whereas it gave Joana 9,300. Certainly this difference in high level quest xp accounts for some of the difference in total time. However, even if you calculate 50 quests at 2k xp difference each, that's only 100k xp, which is less than half a level (near 60), and which amounts to a couple of hours of played time, at most.
I contend that my run is still significantly faster than his.