Significance The European XFEL is a hard X-ray Free-Electron Laser (FEL) with pulse repetition rates in the megahertz regime. Thus, sample dynamics in the microsecond and submicrosecond regime can be accessed by means of X-ray photon correlation spectroscopy (XPCS). Application of XPCS at FEL sources has been challenged because of the intrinsic fluctuations of FELs and the radiation-induced sample heating and damage. In this work we demonstrate the unprecedented stability of the European XFEL and show the feasibility of microsecond XPCS on a model system. Nonlinear heating on microsecond timescales is observed reaching superheated water states that persist up to 100 μs. Our data indicate a violation of the Stokes–Einstein relation for nanoparticle diffusion at short times.