Does performance decrease the higher you climb from sea level?

If someone climbs from sea level to say a shallow 500m - 800m of elevation on a particular hill will there be a decrease in performance as you go up? Also will there be any benefits in doing so? I know the literature goes into detail about high altitude training at 2000m of elevation as the minimum but I want to know if there are any changes climbing from sea level up a mountain.

I’m not sure where reduced oxygen kick in but if the ramp up to 800m is long enough you’ll simply tire.

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Not much drop at 800m but there is some. It also depends on how long you have been at altitude. (my app) can plot power adjusted for altitude: