Phase and group velocity of wave
The frequency cannot change.

What happens to the speed and wavelength depends on what sort of waveguide it is.

If it's a TEM waveguide, like coax or microstrip, then the phase and group velocity fall, and the wavelength reduces.

If it's a TE or TM waveguide, a metal tube with space in the middle, what we would conventionally call a 'waveguide', then the group velocity falls, but the phase velocity increases, increasing the wavelength.

At the waveguide cutoff frequency, phase velocity becomes infinite, and group velocity falls to zero.