Redshifting slows down the frequency of light. But the total amount of oscillations of the light signal isn't affected.

Suppose a far away galaxy sends a 1 second long burst at 1 GHz, 1 billion cycles. At Earth it arrives redshifted to twice the wavelength, and a frequency of 0.5 GHz. We still receive 1 billion cycles, they're just spread over 2 seconds of time now. So we effectively see the signal - and everything else from that distant galaxy - at 0.5 speed.