The ultrafast dephasing of waveguide-plasmon polaritons in metallic photonic crystal slabs is investigated in the femtosecond regime by second-order nonlinear autocorrelation. We find a drastic modification of the dephasing rates due to interaction between localized particle plasmons and optical waveguide modes and subsequent modification of the photonic density of states. In the strong coupling regime our measurements give clear evidence for the appearance of ultrafast polaritonic beat phenomena. All experimental results agree well with theoretical simulations based on a coupled damped harmonic oscillator model.