Kdenlive/Manual/Effects/Motion/Speed
Motion Effects - Speed
Make clip play faster or slower. Use of this effect mutes the audio of the clip.
The Stroboscope setting defines the number frames the effect skips when playing back. For example, if Stroboscope is set to 5 then the effect will only show every 5th frame but will show these frame for 5 times as long producing a jumpy stroboscope effect.
It has been reported that the speed effect does not work very well on H.264 formatted source video. It is recommended to transcode your source material into DNxHD format and apply the speed effect to that. (forum post)