JKL keys: Press 2xL the speed jumps after some time to 1.1 -> press L again same speed for 1.5
They depend on the codec you use. For audio clip only, all the not standard speeds are super-fast and unusable. For the video, some codec respond better than other and the backward speed is frequently very slow or unusable. I suggest trying to fix the speed by linking the playback to a framerate and by allowing the frame dropping.
This is an MLT issue - speeds above 1 are not really reliable. This has to be handled in MLT...