mpv stepping through frames

When using mpv to step through frames (. for stepping forward and , for stepping backward), the forward stepping rate is much faster than the backward stepping rate. The forward rate is about the same as the video frame rate (60fps for the video in question, which is an mp4 file prepared from a set of png images using ffmpeg with libx264 encoding) whereas the reverse stepping rate is about 1fps. Is there any way to control the stepping rates? Why would the forward/backward rates be so asymmetric? If an alternative video player is available for linux systems that supports variable (or at least equally fast) video frame stepping, please advise (mplayer is not better than mpv in this regard).