  Time-resolved diagnostics at Free-Electron Laser (FEL) facilities, in
particular electron beam longitudinal phase space (LPS) and FEL power profile
measurements, provide information highly valuable for users, machine
development studies, and beam setup. We investigate the slice energy resolution
of passive streaker setups, in particular the effect of an energy chirp on the
measured slice energy spread. Downstream of the hard X-ray SASE2 beamline at
the European XFEL, these measurements are enabled by a single-plate non-movable
passive wakefield streaker, essentially a rectangular corrugated plate placed
inside a vacuum chamber. We show measurements with a time resolution down to a
few femtoseconds, and an energy resolution down to a few MeVs.
