The European X-ray Free Electron Laser (XFEL.EU) under construction will provide highly brilliant soft to hard X-ray (<280 eV - <20 keV) radiation with an intra-bunch train repetition rate of 4.5 MHz by employing the self-amplified spontaneous emission process. The resulting statistical fluctuations of important beam characteristics makes pulse-to-pulse diagnostics data of the photon beam a mandatory reference during user experiments. We present our concepts of analysing the photoemission from rare gases with a time-of-flight spectrometer for non-invasive, pulse-to-pulse measurements of the photon spectrum and polarization with a special emphasis on real-time processing with a low latency of ≤ 10−5 s.