not a problem. HKG is as easy as it gets (despite the TSA-required nonsense at the gate).

The flight will close 10 minutes prior to boarding I think, check your boarding pass. If you show up 25 or even 20 minutes prior to departure, you will have no problem at all. As long as you beat the cutoff time.

In the US, there is a tendency for all the flights to board insanely early. On top of that, most flights are domestic so an intl flight is often seen as a big deal (mentally at least). Every flight at HKG is international and it's just business as usual. Don't sweat it.

You can show up 15 minutes in advance if you want, or even 10...but that's starting to really cut it close. They will close the doors at the scheduled time..10 mins prior to departure. Double check what it says on your ticket about gate closing time. that's the one that matters, definitely do NOT violate that or you're going to not have a fun day