Why Can Music Feel Faster Without Adding More Notes?

More notes usually make music sound busier, but note density is not the only source of forward motion. Harmonic rhythm is the rate at which chords change, and it is separate from the surface rhythm of a melody or accompaniment. A stream of semiquavers can feel spacious when the harmony changes once per bar. If chords begin changing every beat, or faster, events seem to gather even though the tempo marking has not moved.

Bach's C major Prelude, BWV 846, from Book I of The Well-Tempered Clavier offers a clear example. Nearly the entire piece uses closely related broken-chord figures, so the fingers keep moving and the surface density remains remarkably consistent. Direction comes from the movement of the bass and harmony, the time particular chords occupy, and the concentration of tension as the cadence approaches. If every semiquaver group is treated as an equally important particle, the prelude can become an even pattern with no destination.

The performer need not accent every chord change. First hear where those changes occur, then allow the bass, pedalling and breathing to reveal the larger steps. The hands can remain even while the ear distinguishes between texture flowing and music progressing. Harmonic rhythm matters because speed exists not only in clock-like beats, but also in how quickly relationships change.

https://iastate.pressbooks.pub/comprehensivemusicianship/chapter/6-4-harmonic-rhythm-tutorial/

https://imslp.org/wiki/Prelude_and_Fugue_in_C_major%2C_BWV_846_%28Bach%2C_Johann_Sebastian%29