Hacker News Reader

Yes, he will slow down the higher he gets. What's driving him up is the difference in density between the Helium inside his balloon and the air outside. The atmosphere becomes less dense with increasing altitude, so the density difference, and thus the buoyancy decreases as well.

Not sure about ascending too fast, I'd imagine at high speeds there's a risk of damaging the balloon due to wind.

12.5 miles is 66k feet. He's at 44k.

EDIT:

But yes, he will also slow down as he goes up. The balloon will also change shape and expand greatly in size.

Ok, I didn't realize the number to the right of the clock was meters and not miles. I guess the decimal instead of the comma threw me off.

he will slow down as he gets higher

Yes, and he will stop going up complately (vertically) after the density of helium will be equal to the density of air.