They are faster than irons if you want to put rounds on target/center mass. Put the dot on target and shoot. It's not meant for precision shots.
I disagree. It's a matter of training and indoctrination, but at short range, a dot sight is at best no faster than a well designed set of iron sights.
It's at longer ranges in excess of 50 yards where the dot sight starts to come in to it's own, and then it's in terms of offering a higher degree of precision without having to align the sights.
Let's consider the Uzi for a minute. It's surprisingly accurate for a sub gun and with a dot sight mounted on it, it is capable of shooting 3 MOA groups at 100 yards semi-auto, at least until the barrel gets hot. Without the dot sight, you'll be doing well to stay on an 12" plate at 100 yards given the rather crude iron sights and the short sight radius.
However, at close range, it's hard to beat an iron sighted Uzi for speed.
Those great big front and rear sight ears, the cocking knob and very good pointing characteristics make it extremely fast when snap shooting center of mass at close ranges of 25 yards or less. You literally just raise the weapon to eye level and look over the weapon at the target naturally framing it with the front sight ears with both eyes open and put rounds center of mass. You're not down on the stock looking through the sights but rather you've got your head erect looking over the sights.
It's from 25-50 yards where it becomes a bit of a toss up. A tube style dot sight like this Ultradot interferes with the field of view a bit, but you once again raise the weapon into your line of sight and with both eyes open put the dot on the spot you want to bleed. It's marginally slower, with a slightly reduced field of view, but it is much more precise.
At ranges beyond 50 yards the dot is the clear winner as it offers more precision, at speed, than using iron sights.
I this case with the dot sight mounted above the regular sights, the head position is the same with or without a dot sight attached.
----
Now...some folks choose a hybrid approach and want to use a low mounted reflex sight like the Burris Doctor. It's low profile, light weight and co-witnesses with the iron sights when the mount is screwed or spot welded to the top cover.
The advantage at close range is that you can still look over the top of the sights and with your head erect, frame the target between the tops of the front sight ears - i.e. you are not using the dot sight at all.
At longer ranges however (25 yards or more) you've got to get your head down to look through the dot sight - close to where you'd be with the iron sight, which is actually a little slower, particularly if you are transitioning from a close shot to a long one and back again.