"You see there, PHP running at front with 68 minutes average accepted answer time, either it’s too easy or there’re too many of them."
It's an average, the size of each data-set shouldn't really affect the results. If there's too many PHP questions then answer times would likely be slower. Seems just like an excuse to dig at PHP.
I think he means that duplicates might be causing a problem? PHP is very approachable, and you probably get a lot of new users asking similar questions, which would be handled quickly. I think this was his motivation in creating the plot of 'hard' questions which took more than a day.
It's an average, the size of each data-set shouldn't really affect the results. If there's too many PHP questions then answer times would likely be slower. Seems just like an excuse to dig at PHP.