Work divided by time
Two sessions can contain the same exercises, sets, repetitions, and load yet feel completely different when one uses shorter rests. Density names that difference.
The useful comparison is with your own prior session under the same standard—not with someone else's stopwatch.
Do not confuse density with haste
Rep cadence, setup, and range of motion remain fixed. Only the unproductive space between completed sets is reduced.
If shorter rest forces partial repetitions or careless transitions, the number improved while the training worsened.