If you download the same file twice, your OS and browser will aggressively cache it. Use curl with --header "Cache-Control: no-cache" or test in private/incognito windows.
ffmpeg -f lavfi -i testsrc=duration=600:size=1920x1080:rate=30 -f lavfi -i sine=frequency=1000:duration=600 -c:v libx264 -b:v 14M -c:a aac output_test.mp4 Use code with caution. Best Practices for Running Video Tests download sample mp4 video files for testing 1gb new