It depends what you're doing, there are many different ways to cut a section out of a video. If you do it copying the streams (not reencoding anything) ffmpeg s

It depends what you're doing, there are many different ways to cut a section out of a video. If you do it copying the streams (not reencoding anything) ffmpeg should do it at almost the speed that it can read and write to disk. However if you are reencoding the video, the speed will depend on every parameter controlling the encoder. If that's what ScreenFlow is doing, it's probably using hardware accelerated encoding too.