This situation requires DM adjudication.
You don't actually say it, but it seems your implication is that RAW, in your scenario each creature's action happens within the single round and therefore as the value of n increases, the speed of the incapacitated person increases.
There are a bunch of situations you can come up with that abuse the idea that the game world is simultaneous, but the table is turn-based. The only real limit on this is what the DM will allow.
First of all, your question assumes that the action economy is the best way to resolve the scenario. But according to the rules, "The DM might use a different time scale depending on the context of the situation at hand."
Assuming there is time pressure and that therefore the action economy is the best way to arbitrate the situation, you are postulating that n characters can each sequentially execute an action A that takes time t and that no matter how large n becomes, n * t will never exceed a single 6-second round. Essentially, you are postulating that time t is 0.
The maximally cooperative DM
The theoretical maximally cooperative GM will agree with you, that time t is 0, in other words, an infinite number of creatures can hand off the incapacitated creature one after the other within a single round and therefore as the incapacitated creature travels across the arbitrarily large field in a single round, it's speed also becomes arbitrarily large.
Of course, the theoreticallly maximally cooperative GM may also rule that long before the speed of the incapacitated creature reaches any theoretical maximum imposed by the likely unstated physical limitations of the game world, air friction causes the now incandescent remains of the incapacitated creature to tear a hole between the prime material plane and the plane of fire, engulfing the arbitrarily large field in an arbitrarily large fireball.
Or maybe air friction isn't a thing either, and you've just invented high-speed transportation.
Or maybe the above is actually the jerk DM, and the maximally cooperative DM just shrugs and says, yeah, you get the incapacitated creature to the other end of the field.
In my experience, a lot of GMs will look at you and either shrug and say, "sure, I'll allow it", or say "shenanigans, hard no".
What the rules say
The rules say:
In situations where keeping track of the passage of time is important, the DM determines the time a task requires.
The DM must determine time t.
If the DM determines that time t is 0, then yes, action A can be repeated an n (or infinite) amount of times in 6 seconds.
The DM may choose to make a different determination. This is explicitly the DM's job.
The fact that the rules do not explicitly spell out what time t is, does not mean that the rules as written assume it to be 0. The rules explicitly leave it up to the DM to decide.
How does the DM decide?
Since this requires DM adjudication, there are as many ways to decide as DMs.
I don't think your question is really about how long it takes to hand off an incapacitated person from one person to another, but based on real-life experience and guesswork a DM could say, hmm, that will take about a second for each handoff, therefore you can move the incapacitated person 60 feet per round.
Also, see the question:
Are there any limitations on the length of a chain of actions
and the answer:
Yeah, the limitation is the DM