Most letter pairs are shared between two fingers, often between two hands, so they come out almost together: the second finger is already on its way while the first is still pressing. Some pairs, though, land on one and the same finger, and then there is nothing to overlap. The finger has to release the first key, travel to the second and only then press.
e and d i and k o and l
These are every typist's slowest spots, and usually the very places where a whole line stalls. It is easy to measure: “minimum” and “school” come out noticeably slower than words of the same length whose letters alternate between the hands.
One thing helps: move the finger, not the hand. When the hand follows along, the other fingers have to find their places again afterwards and one slow spot stretches into three.
Beyond a certain point these will not get any faster, and that is the right expectation. The goal is that they stop stalling, not that they match the rest.
The drill gives the pairs on their own, with no word around them. In real text you meet them inside words: “minimum”, “junior”, “united”, “school”.