After thinking a lot i noticed that the speed in music has two aspects as i see it. I try to explain how i am reasoning.
We keep us to the four quarter notes, 4/4 time signature
The first option is how long every and each quarter note is sounding. If we think ordinary tango with 5 parts, 16 measures each, then we get totally 80 measures which usually takes 2,5 mins to play. But we could modify the music file and play it faster, in 2 mins or slower so it takes 3 mins.
How many measures are passing per minute?
Another way to create a feeling of speed is the number or accents within a measure demanding our steps.
How many accents there is per measure?
As dancers we can decide the number of accents we want to use for our steps. Usually in tango we have two accents per measure. If we would step only on 1 and the same for next measure we create a much calmer, slower feeling. In milonga there is 4 accents and when we take them all it can go quite fast!
How many accents we step per measure?
When you are playing a dance video in slow motion you are using the first alternative. The music and dansers steps are the same but sloooooower.
In dance the steps are the changing element and the music a constant