Heading uphill forces you to slow down and alters the ideal run-walk transition. For most people, walking is more efficient than running at slopes greater than about 15 degrees (equivalent to a 27 percent grade). For elite runners, running might still be more efficient at somewhat steeper slopes, but eventually everyone reaches a point where walking is the most efficient option. Note that "efficient" and "fast" aren't the same thing: if you're just climbing a short hill, you might still choose to sprint up it even though walking would be more efficient. But if you've got a long race ahead of you, efficiency matters—and most mountain runners have had the demoralizing experience of grinding up a steep incline then noticing that the person beside them is climbing at roughly the same pace even though they're walking.
https://www.outsideonline.com/health/training-performance/uphill-trail-running-research-2022/
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