Anectdotally from my experience, I don't feel any altitude effect below 5,000 feet (not saying there isn't one, but its not enough for me to notice).
I live at 15 feet above sea level(when the tide's out), I'm from WV and ride at 4500 feet there each and don't notice it. Was on the Blue Ridge Parkway this weekend at 3400 feet and no noticeable effect.
First time I did Everest Challenge, I thought my pacing was off on the climbs because I started hurting about half way up, After a few repetitions, I realized that it happened at the same height each climb. For me about 7500 feet is where I notice a drop off.
Two weeks from now, I'm doing the Leadville 100MTB, coming from sea level to race between 9,000 and 12,000 feet above sea level.
That will hurt.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.