As for numbers, well, I can give you a decent enough ranking on time to aim and best possible aim.
Top speed will be pistol, yes, but it will also be the lowest level of accuracy as well.
Next for speed (maaaybe 2x pistol?) would be red dot, followed closely by holographic (maybe 1/2 pistol? much less than previous step), and eventually by iron sights (2x red dot wouldn't be too bad for someone skilled).
Bringing up the rear is of course the scope. Hard to say how fast, since different zoom levels require different amounts of time, and a scope with a high minimum magnification takes a different approach to get an initial target. Super rough generalization? 1 1/2-3x iron sights. Up to triple that if the shooter has no experience with scopes however.
As for minimum level of inaccuracy, these are going to be a bit fuzzy, since the difference between the point of impact and the natural point of aim will matter more than actual aim when it comes to an unfamiliar weapon. (This is what calibration and zeroing are for)
Almost as problematic as (or maybe more than) calibration is level of familiarization, with the specific weapon, the weapon style (carbine, bolt-action rifle, full or sub-compact pistols, etc.), and just basic weapon handling. Trying to be more precise in aiming is going to be exponentially worse for a new shooter than it would be for an experienced one. (level of inaccuracy sharply decreases with basic levels in the mentioned variables.)
Lowest MoA will be scope of course. It probably has very low double-digit arc seconds of dispersion for a well calibrated and familiar weapon under ideal conditions (wind would have more of an effect than any error of the shooter's aim). An average shooter in average conditions could probably manage to keep it under 2 MoA with some practice though.
Next, I would say holographic sight in a scavenging situation, as it's already calibrated to the barrel, and the holographic nature helps correct poor sighting by the shooter. Average for this would probably be in the low double digit MoA dispersion. (35ish?) Ideal conditions would probably be half that level. However! Maximum aim bonus from time would be sharply decreased after a couple seconds (same for red dot and pistol) because these sights are designed for speed, not accuracy.
Next is Iron Sights. They would rank higher than holographic if the person was already familiar with the weapon, but lower for someone's first few times firing, as it's dependent on the shooter much more than the holographic ones. (harder to know you have a proper sight alignment for the weapon.) I would say you could get this into the high single-digit MoA range for best conditions. Average conditions would probably be 25ish.
With Red dot sights, there's basically no sight picture correction, so you don't really have feedback as to what angle the sights were calibrated to. The dot also somewhat obscures small targets, leading to greater inaccuracy at distance. Average inaccuracy is probably around 50-60 MoA. (Ideal maybe 40)
And finally pistol sights. There's a reason they're only used in short distance shooting. Minimum inaccuracy for even the best shooters will be (slightly) above 1 degree. Average shooters would probably be in the mid single-digit degrees. Fully unfamiliar or improper training (gangsta style
) will be over 10 degrees at minimum.
I can go even further into this particular aspect, but I'm trying (and failing I'm sure) to keep it within the scope of the discussion.