What do you mean by "historically fastest"? Do you really expect a course record set every year? Just because no one beat last year's first place time doesn't mean everyone went slower; if you're just looking at the fastest run as an indicator of speed, I think you're missing part of the picture.
2014 Women's top 4 average: 2:44.04
2015 Women's top 4 average: 2.42.49 (-1.55)
2014 Men's top 4 average: 2:13.35
2015 Men's top 4 average: 2:13.04 (-0.31)
I think what you're actually observing is that a previously dominating team has lost some ground this year, and the rest of the pack is closing the gap. The top times are objectively faster overall than last year (regardless if this is happening in freeroll or pushing), and the difference in those times is smaller.
Sam Swift did a excellent data analysis of historical data (I don't have a link handy for the slides at the moment), and I am pretty interested to see how this year affects the dataset. One of the thoughts I had was to try and break it down further between freeroll and pushing, but I'm not quite bored enough yet to sign up for watching a ton of race footage to get those numbers.