Why does a round last 6 seconds? [closed]

There are many posts and discussions about what can be achieved during one round, however, to the best of my knowledge there is no explanation as to how/why the number 6 was decided.

The question becomes more interesting considering that all of the RPGs I have played so far also consider each round to last 6 seconds.

I think it’s worth breaking this question into smaller ones:

  1. Who invented the 6-seconds round rule? Is it Gary Gygax?
  2. Why 6 seconds? For example, why not 5 or 7 seconds. In particular, 5 looks nicer and "rounder" to me.
  3. Are there any RPGs in which rounds last shorter or longer? Do they offer any explanation as to why?
  4. Does the game break if you change round duration? Let’s say that I make a house rule that rounds last 5 seconds. What’s going to happen?

p.s.1. I’m not sure which tags are most appropriate here. Please, feel free to suggest any for adding or removing.

p.s.2. I’m taking D&D as the baseline and assuming that all RPGs use the 6-seconds rule, this is why I’ve tagged the post with the dungeons-and-dragons tag.