How does the speed of the density wave compare to the speed of stars/gas in the disk?

The wave moves at a different speed than the stars and gas

A key distinction defining the nature of the density wave, as opposed to a static material pattern, is its speed relative to the components orbiting within the galactic disk. The density wave itself propagates through the disk at a specific rotational velocity, which is distinct from the orbital velocities of the individual stars and clouds of gas and dust. Because the wave moves at a different speed, material entering the high-density region experiences temporary compression, triggering star formation, and then the material moves out of that dense region as it ages, continuing its orbit at its own speed. This speed differential is crucial for the continuous turnover of stellar populations within the spiral arms.