I'm pretty sure that the long waiting times are more likely due to the influx of Tier 4 visa applications than the priority service applications - the US consulates deal with, on average, 1,500 applications each month in total (for all types of visas).
However, looking at the stats for August, the LA and NY consulates have processed about 1,500 Tier 4 visas alone (Chicago had about 800), which means that there were double the number of applications being received at each consulate in August... it only makes sense that the processing time for settlement visas will just about double as well, especially as those 1,500 Tier 4 applications are likely to have been given priority over the settlement visa applications due to the students having to travel by September.
Now that the Tier 4 visas will have mostly been processed, there'll be a backlog of other types of applications that they now have to get to, which again is probably why it's taking so long.