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 efect will only show every fifth frame but will show these frames for five times as long, producing a jumpy, stroboscopic 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 the DNxHD format and apply the Speed effect to that. (forum post)