A more open sight would be less accurate, and probably slower to acquire. All sights are a trade off, no one design is perfect for everything. Open sights main "pro" is that they are cheap and very durable. They aren't particularly accurate, are either non-adjustable or minimally adjustable, and aren't very fast to acquire a sight picture with (with some notable exceptions).
Peep sights (aka aperture sights) are both faster (generally) and more accurate. They also offer a trade off, though. The bigger the disk in the rear, and the smaller the hole, the more accurate it is. The smaller the disc and the bigger the hole, the faster it is. The most open type is a ghost ring, which is very fast to get a sight picture with and to acquire a target with. However, the larger gap all around the front sight makes it harder to judge and get it centered on both horizontal and vertical planes. The M-16 still has peep sights, and the rear sight flips to give you a big aperture and a little aperture. For close quarters, you flip to the big aperture because you want the speed to start putting rounds on a target as fast as possible and are more worried about hits than bull's eye accuracy. At longer ranges, you need the extra precision afforded by the smaller aperture despite the speed penalty. IIRC, the Army trained us to flip to small past 50m unless it was dark and then you always use the large aperture because you need more light coming through.
Now, there are exceptions to the rule. There are some very fast open sights out there. They tend to be horrible on accuracy, though, usually with a very wide and shallow rear "V" type notch and a large ball of a front sight. These are often on "bush guns", or rifles made to take down dangerous wild game in a sudden encounter. Think of turning the corner and a pissed off Grizzly is 40 yards away and starting to charge you. You've got a really big target that you need to dump rounds into really fast, so that type of sight makes sense for that type of rifle.