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In iMovie you can change the speed of a clip by selecting it in the timeline and then clicking on the icon that looks like a speedometer: Then click on the "Speed:" popup menu and choose "Fast" or "Custom". The "Custom" setting lets you choose how much faster to make it (like 200%, or 50%).


You can use ffmpeg, a command line program, to double the video speed: ffmpeg -i input.mp4 -vf setpts=0.5*PTS output.mp4


The videos I've delivered for iOS App Previews have been as per the specification in this section, viz. H.264 video codec, Baseline profile, 30 fps, AAC-LC audio codec, sampling rate 48 kHz, stereo in MP4 or MOV. Resolution and bitrate haven't been restricted to what the docs say. Those should be tailored to the device(s) your app is for.

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