scopes
The old saw about irons being faster than scopes likely goes back 50 years or more. Fields of view were smaller, the rifle scope was still a bit of a novelty, and almost everyone had learned to use irons first, then moved to a scope, and the correct technique to run glass quickly was not as well known .
Scopes were not as reliable either, and the goofy Ironsighter rings and swing away mounts were popular for a reason, your scope might well just go sour at the wrong time.
Scopes, even good ones, can still go bad, and quality detachable mounts, and a functional set of irons are still a good idea on a hard use rifle. But a modern GOOD scope is a pretty durable item. And yes indeed, there are fragile irons, the factory sights on most US sporters and slug barrels are not as tough as you might think. Replacing lost and bent bead and blades is a common task for those who tinker with repairs.
There are some variables w/ quick use of a scope. One is magnification level. The more "X" power you have, the harder the target is to find in the smaller field of view that goes along with high power.
Another is tube diameter, and certainly bell diameter. No proof of this, but I suspect that the slightly bigger view in a 30mm tube aids to a SMALL degree in quick use. Their are more technical reasons for a big tube, but improved FOV is one. Consider the demise of the narrow 7/8 and 3/4 inch tube and the near universal adoption of the 1" tube for general use. And 30mm was next, and they are getting bigger. The big tubes and eye boxes make for a larger window/field f view, but.........
Big bells bring their own problems however. The big bells and high X power of the current fad, mean scopes mounted higher on the receiver, which equals a poor cheek weld, sloppy rifle mount and reduced field of view. Big scopes are heavier too. Your Grandad's or great uncles 1 inch fixed 4x mounted low on the rifle, was likely much faster than the current big glass so popular with some.
Another aspect is reticle choice. Many of today's are to fine, difficult to see and complicated. I believe the duplex is about as busy a sight picture as one needs for quick shooting, and strikes a good blend of visiblity and precision. The old post and crosshair (coarse) or the German #1 are likely best suited fast shooting and low light, but these patterns are not common today, special order at best, and fading fast. They are not best for long range work either, which is the current fascination.
I hate to bring it into the discusssion, but since we're talking speed, the red dot is likely the fastest set up, it certainly is for me. The dot seems to be quickly taking over the role of the basic iron sight. Shades of Star Wars.
Finally, some folks tend to peer through the scope with one eye, which only uses half our optical system, and is slower. The natural tendency is to let the master eye take over and close the off eye, and look through the scope. But I encourage anybody looking for speed to work a bit with both eyes open and see if they improve. For many folks, it helps.
There are folks who can do very good work with irons/peeps. And there are some iron set ups that are extremely durable and proven. But I believe that a wisely chosen scope, properly mounted and practiced with, will be superior in all aspects of shooting for most of us.