There are two big differences between discs and deep section wheel and its the combination of those drive the use of discs. Along the same lines all discs (and even disc covers) have these two advantages which is why you don’t expect big differences between discs.
In terms of straight line speed discs are faster than any other deep section design because they have a low ‘watts to spin’. This is the drag caused by the spokes moving through the air. Watts to spin is also why tri-spoke wheels are very fast. This is not something you can measure in a wind tunnel and so its hard to quantify on charts but is very real.
The other very real benefit of discs is the fact that in the real world they won’t ever stall out aerodynamically. This is a fancy way of saying they handle in a very predictable and stable manner and in cross wind situation they are noticeably superior to deep section wheels. Under extreme conditions the stability of discs is a potential problem because it means they are very determined to consistently blow you off course. Kona is the one case where there this occurs with sufficient regularity to ban the use of disc but the general consensus is that disc make bikes easier to handle.