Likelihood of needing to immediately fire to defend yourself...
It goes way beyond just speed.
1. The likelihood of not having a free hand to rack the slide due to injury or having to hold a child or drawing from an unorthodox position, or having to defend with the weak hand, etc.
2. The likelihood of inducing a malfunction when racking a round into the chamber. As mentioned, manually chambering a round was less reliable than the normal firing cycle in the ISHOT1000 matches.
3. The likelihood of needing to ready a firearm surreptitiously. The normal loading cycle of a semi-auto pistol is quite loud.
4. The likelihood of being injured in the arms or hands in a way that makes it difficult to manually load the chamber.
....the likelihood of a negligent discharge of an always-chambered round in a cocked pistol…
1. There are many firearms out there that can be carried chamber loaded without being cocked.
2. There are many firearms out there that can be safely carried chamber loaded, or even chamber loaded and cocked.
3. If a gun is handled properly, there's no need to significantly increase the chances of an unintentional discharge by having the chamber loaded. One can take one's time and choose the time and place to deal with a chamber loaded firearm to insure that there's no rush and there's no stress that could affect the outcome.
There's really no hard and fast right answer.
No, there's not. But there is an answer that's right for the vast majority of people out there.
There are two solidly defensible reasons for carrying chamber empty:
1. It is mandated. The person is given no choice
2. There is no availability of firearms that can be carried safely with a loaded chamber.
Other than that, it gets really problematic to justify chamber empty carry.