Wheel speed and fan speed are not related (unless they have some super fancy hookup).

Without knowing the speed our guessing is more useful - other than just trusting them saying the Venge is faster than round tubes.

The difference is huge on decent. I have video of my kid (early 15) chasing his coach. Coach on a Venge, kid on a TT bike. At that time they were roughly the same speed on the flats and 40lbs different weight. My kid descended 37+, coach was 34ish. Time difference was 3:15 in 25 min and much due to the climb too and the heavier rider, but as that course is ridden some 30X a year, its pretty clear to us the TT bike (mostly wheels) is much faster.