Look around. Look where instantaneous is used. It's very specifically a term used in very specific places.
Yes, it's used in spells, as per the definition that I posted, meaning "in an instant". This is the RAW.
So you honestly think people in combat walk at the speed of teleport when they ready an action? This is how you run your games to make sense?
What I'm saying is that over a short distance, it might be as fast for a combattant to just hurry there in a flash than waiting for a star trek-like teleport where you rather slowly fade out and in, with possibly some time in the transfer itself. Both of these happening "in an instant".
Just out of curiosity, if you're okay with the ridiculousness of walking/running faster than a teleport to avoid something happening at the speed of sound
How fast is the sound in your game world ? Is it always the same one ? Is it influenced by air pressure ? Because, that in turn, depends on the gravity and we
know that gravity is not that of the real world in D&D, see for instance Spelljammer gravity which is official in 5e.
There are a lot of things that can work very differently. Moreover, the speed of sound might not even be relevant, if the fade out is relatively slow, the boom might allow enough time to roll out the the way, 15 feet is honestly not that far at all, even at my age I can manage a judo lunge, fall and rise to be more than 15 feet away in a blink.
There are so many explanations, like the fact that, although it's in an instance, it's not really a sonic boom damaging people (it would be silly for it to be in a cubic shape after all, and to stop abruptly out of that cube), maybe it's just magic creating a boom inside the boundary, and that part of the magic needs an instant to build up, giving time for someone to roll out of the way.
In any case, it very cinematic.
why do you have an issue with the ridiculousness of sequential nature in D&D combat as written?
First, it's not written as sequential, it's written as simultaneous (cf. the snippet that I sent you a few posts back). The resolution of the turns is sequential, but I hope that you are not still advocating that every combattant is freezing in place until it's his turn to act ?
This is why I don't have a problem with it at all, and why it poses me no difficulty whatsoever to have globally simultaneous action with sometimes sequential events influencing the whole narration.
But it causes YOU a problem, possibly with the freezing problem above, but with the duration of the rounds, since you are pretending that turns are all 6 seconds long like the rounds, which would cause the rounds' lengths to be not "about six seconds" but six seconds times the number of combattants...
Sure, but the amount of time is so incredibly small that there wouldn't be time to move in-between the disappearance and re-appearance, and would be faster than the speed of sound.
See above, you have no proof that disappearance, or reappearance, or transfer, or boom, or whatever is that fast. The only thing that we know is that it happens in an instant that is too quick for it to be caught by a dispelling. And if you are discussing the RAW, all your other personal preferences and hypothesis are irrelevant.