I'm executing basic ffmpeg command to transcode input MP4 video to output MP4 video (both using H264 as video codec, and stripping out audio, for simplicity):
ffmpeg -i input-video.mp4 -b:v 20000k -an -vcodec libx264 output-video.mp4
Transcoding is initially pretty fast (around 60-70 fps for first 20 or so frames), and then starts dropping steadily, ending at around 30 fps after 5000 frames.
Is this inherent behavior? If so, why does it happen? Also, is there any better option for the command line that would improve speed (besides using -threads)?
Would it be better to cut the input video and process it as smaller chunks, getting overall higher speed per sequence? (I'm afraid that chunking might affect encoder's RD optimization)