In my "experience" the rolling resistance of the wheelset and and ability to transfer energy to the sheelset are the two most important items in mountain biking. Then of course you have to have a decent rider. Thus my favorite saying-

"Its the Indian, not the Arrow."

I have a couple of older HTs' One is a Deore setup, the other an XT component group. I tracked (Garmin 305) myself on similar trails on alternating identical rides for a month. The better the components the better the higher average speed. SUBJECTIVELY speaking, the better rims and hubs helped maintain speed and required less effort on climbs.

The better Hubs, rims, tires, bottom brackets also happen to be lighter and more expensive.

On the other hand, the energy expended in riding my weight down to 215 from 225 has made a big difference in the "Indian operating the arrow."

Now I have a F/S bike to learn all kinds of new stuff on.