It seems like, in my area, that in the last few years a lot of roadies have been trying out MTBing. They are very fast on the connector trails and hills that are not too steep (traction issues not conditioning issues), yet really slow on the highly technical trails and the downhill. This is a generalization, but is meant more for roadies that only MTB because some of there friends do, versus folks who like or love both road and MTB riding.

I have talked with a ton of these guys over the past two years. For the most part, they are just too tense when riding the areas that they find difficult. Like others have said, relax, ride more, and watch some of those videos.