I've never done a cross race but I did some mtb races. The big difference is the "minimum power required to move" bit, i.e. rolling resistance. On the road it's a negligible number; when on dirt or on foot it's a significant portion of your total energy output. Therefore once you reach your "I am going as hard as I can" level for a given surface then you're going to be limited to that speed, regardless of the draft etc.
On the road rolling resistance is relatively insignificant. The biggest limiter is wind resistance and drafting goes a long way towards eliminating a huge portion of that resistance.
__________________
"...during the Lance years, being fit became the No. 1 thing. Totally the only thing. It’s a big part of what we do, but fitness is not the only thing. There’s skills, there’s tactics … there’s all kinds of stuff..." Tim Johnson