Explanation
Most pacing problems are visible in a shot list before anything is made. A three-second shot in a fifteen-second video is a fifth of the entire attention budget, and seeing durations written down makes an overlong setup obvious in a way reading the script never does.
The workable rule is that nothing should sit still for more than about four seconds without a change, and no single idea should take more than one shot to land. Where two ideas share a shot, one of them is not landing.
The common mistake
Fixing pacing by speeding everything up. Uniformly fast is its own kind of monotony, and the videos that hold attention vary their rhythm rather than maximising it.