Fear/anxiety helps climbers perform better in the short term such as in competitions:
"while fear may be helpful in that sort of short-duration exercise (it can keep the mind sharp and help climbers avoid accidents)."
But on multi-day climbs, fear gets in the way of success.
"Perhaps not surprisingly, those with high levels of anxiety did not, on average, make it as high up the mountain as those who reported they were relatively calm."
The researchers studying this, "speculate that climbers with that mental attitude 'are energized by the attainment of each smaller goal en route to the summit (e.g., successfully crossing a challenging crevasse), and this helps to fuel continued engagement.'"