Ok, I still don't see the problem. If the DM chooses to have the sorcerer cast without visible wind up, that's just how casting works in this world, and the player looking to disrupt the magic needs to find out how after learning this.
Just like real life. If there was magic.
Unless they have reason to know how fast casting is beforehand. Of they do, then sure, they should know those rules.
Ideally, the player should at least play their PC using only information the PC has reason to know. IMO of course.
Again, just like real life. If there was magic.
Because there's a lot more to combat than who goes first.
Stop motion initiative is a necessary abstraction for the system to work (at least to my liking; I find simultaneous initiative extremely annoying). Like hit points. It's not perfect, but I like most alternatives less.
So is stop-motion resolution a "necessary abstraction"? Or is it a model of how fast casting is in the imaginary world?
In the imaginary world, the sorcerer begins speaking the vile words and making the arcane gestures, and the doughty barbarian, standing on the other side of the room, rushes across and interrupts the casting.
At the table, is this resolved via declarations that unfold in the same time sequence as the fiction? Let's say, the GM chooses a spell, declares the sorcerer NPC starts casting, and then the player declares that their warrior PC rushes across the room to interrupt, and we resolve via opposed Speed checks. If the NPC wins, the GM's chosen spell comes to pass.
Maybe the approach to some part of the time sequence is the same, but not all: the GM declares that the sorcerer NPC starts casting a spell (but has no particular spell chosen - maybe the system doesn't even have "particular spells"), and the player declares that their warrior PC rushes across the room to interrupt, and we resolve via a check (let's call it Defy Danger) on the PC's Speed - if the check fails, then the GM is at liberty to bring home the spell effect, maybe by reference to some general principles of play and some particular details about the goals and inclinations of the NPC.
Or maybe this little scene is resolved via a system of prepared interrupt actions that unfold in a sequence different from the time sequence of the fiction: let's say, at the top of the initiative order the player declares that their warrior PC is ready to interrupt any spell casting, and then the GM makes a complete action declaration for their NPC, declaring that the sorcerer is casting a particular spell, and then the player resolves their readied action, getting a roll to interrupt the casting and thus negate the GM's action declaration for the NPC. If they fail, the GM's action declaration stands.
These might all be the same fictional world - let's say, the world of Conan and Thugra Khotan - but what the player needs to know about the rules of play, in order to bring it about that their doughty barbarian PC interrupts the sorcerer's attempted spell, and what the player is risking by attempting to do so, is different in each case. In the first case, the GM is bound early as to what happens if the spell is successfully cast, and the player can respond in real time. In the second case, the player can respond in real time, but the GM is not bound as to the consequence of the spell-casting until the player's response is resolved. In the third case, the player has to choose to attempt interruption, becoming bound in that respect, before the GM says anything at all about what their NPC is doing.
These differences have nothing to do with things being like, or unlike, the real world. The differences are entirely about how the rules of the game manage the unfolding of the fiction in response to the participant's contributions.