If you have mov video and want to be able to start playing without having to first load the entire video, -movflags=+faststart is needed for ffmpeg to put the meta-data, known as the moov atom, at the beginning of the file. Otherwise, ffmpeg puts this atom at the end of the video file which means you have to wait to play until the whole video is loaded. Or worse yet, if the file becomes damaged in the middle and you can not get to the end, you won’t be able to play anything.
Now you can have the moov atom put on the front of the file (automatically via a second pass). To do this, when rendering using ffmpeg & either the mp4 or qt format/container, click on the video/audio wrenches and choose faststart_h264. With the qt format, settings will just be the default whereas the mp4 format uses the highest quality and lowest file size as possible, but you can easily modify these options in the associated Video Preset textbox.
The CINELERRA-GG Community, 2021