Physics: Principles with Applications (7th Edition)

The speed of a longitudinal wave in the Slinky measures how fast the wave disturbance travels along (parallel to) it. It depends on the tension in the Slinky and its linear mass density. Since those quantities haven’t changed, the wave speed is the same. However, the wave speed is also the product of the wavelength and the frequency, $v = \lambda f$. The wave frequency increases, so the wavelength decreases.