How does the slow spell actually work in D&D 5e?
The description states: "You alter time around up to six creatures of your choice in a 40-foot cube within range."
Is this talk about their perception of time, so they see everything moving really fast? Or does it affect the entire AOE space?
If you're altering time I would assume objects could be affected, but the spell mentions only creatures – but that seems odd for the ability to alter time in a 40-foot space.
Best Answer
Spells Only Do what they Say They Do
The Slow spell only dictates that it affects a creature's movement speed, their ability to use actions and reactions, the number of attacks they can make in a turn, and a creature's spellcasting abilities. The spell doesn't specify effects on projectiles or other non-creature objects, so Rules as Written, the Slow spell does not affect any of these things.
Be wary of giving extra effects as a Rule of Fun option
Being able to slow down projectiles would probably dramatically improve the effectiveness of a spell like this, since it would potentially give defenders the ability to avoid ranged attacks entirely, depending on how you chose to rule the effect of such a phenomenon. This would make the spell more powerful than intended, and Slow is already considered to be a quite powerful spell. Other things like slowing down falling rocks could have very significant consequences for the game.
How to interpret the line "You alter time around up to six creatures of your choice"
In principle, there's no difference in 5th Edition D&D between flavor text and mechanics.
In principle.
In practice however, there aren't a lot of mechanics that specifically care about a creature's personal "flow of time", so to speak, and even for features that do, the Slow spell doesn't specify how exactly their personal sense of time is affected, except to specify the specific mechanical constraints laid out by the description.
So it's probably not worthwhile to pick apart the possible mechanical semantics/implications of that line. Stick to what the spell actually says it does, and maybe encourage some roleplaying consequences, like forcing your players/characters to speak in a deliberately slowed down manner, or being surprised by the alacrity of a character moving at normal speed adjacent to them.