Advanced time-correlated single-photon counting (TCSPC) devices are able to record several 106 photons per second and deliver an instrument response function down to 25 ps FWHM. Under these conditions the accuracy of fluorescence decay or photon migration times is limited by systematic timing errors rather than by the photon statistics. The experiments described below determined the variation of the instrument response function (IRF) with the count rate and the timing drift for an SPC-140 TCSPC module and a number of commonly used detectors. For count rates from 3×104 to 4×106 s-1 a shift of the first moment of the IRF smaller than 2 ps was obtained. The drift over 16 minutes was within ±0.7 ps.