What procedures are followed during opening of the hatches of both ISS and Crew Dragon, and why does it take so long?
As I understand the livestream, the most important thing was pressure equalisation and subsequent leak checking.
I guess this could be done faster, but it's just not worth taking any risks on it. Imagine there's some problem with the docking securing. As long as there's no pressure between the hatches, this wouldn't be seen (no mechanical load). Now as you add pressure, this works to... well, press both spacecraft apart. Maybe even the faulty joint would briefly withstand this, and at first only leak some air. But if it then breaks after the hatches are open you have a huge disaster. If it breaks with the hatches still closed... well, you'll probably have a mission abort and the capsule has to return to Earth. Annoying but not catastrophic.
Anyway the crews generally have lots of stuff to do at that point, like getting the audio connections up running, ...cleaning the toilet...
The main concerns, in the order they occur (but in increasing order of slowness of waiting)
- pressurization and leak checking, then
- temperature differences (from gasses changing pressure as it's filled in) leading to possible additional leaks.
Then one side is opened and:
- dangerous gases air out - in zero-g they don't mix very quickly, and tend to accumulate in pockets, such as carbon dioxide, carbon monoxide, or contaminants such as HCl (hand-held gas detectors are used).
For the Demo 2 docking yesterday, the Node-2 hatch was opened hours in advance, to allow it to air out (but this wait was a big delay in the Demo 1 docking); but the docking hatch cant be opened until docking is sealed, on the ISS side was opened and aired out for, I believe it was 1 hour, before the dragon hatch was opened and people moved through it.