why does moviepy's write_videofile function get slower and slower as it processes each frame? And how to improve/fix it?

Question

My script takes two movie files as an input, and writes a 2x1 array movie output (stereoscopic Side-by-Side Half-Width). The input video clips are of equal resolution (1280x720), frame rate (60), number of frames (23,899), format (mp4)... When the write_videofile function starts processing, it provides an estimated time of completion that is very reasonable ~20min. As it processes every frame, the process gets slower and slower and slower (indicated by progress bar and estimated completion time). In my case, the input movie clips are about 6min long. After three minutes of processing, it indicates it will take over 3 hours to complete. After a half hour of processing, it then indicates it will take over 24hours to complete. I have tried the 'threads' option of the write_videofile function, butit did not help. Any idea? Thanks for the help.

---- Script ----

movie_L = 'movie_L.mp4'
movie_R = 'movie_R.mp4'
output_movie = 'new_movie.mp4')
clip_L = VideoFileClip(movie_L)
(width_L, height_L) = clip_L.size
clip_L = clip_L.resize((width_L/2, height_L))
clip_R = VideoFileClip(movie_R)
(width_R, height_R) = clip_R.size
clip_R = clip_R.resize((width_R/2, height_R))
print("*** Make an array of the two movies side by side")
arrayClip = clips_array([[clip_L, clip_R]])
print("*** Write the video file")
arrayClip.write_videofile(output_movie, threads=4, audio = False)  

Answer 1

I have also had problems with slow rendering. I find that it helps a lot to use multithreading and also to set the bitrate. This is my configuration: videoclip.write_videofile("fractal.mp4",fps=20,threads=16,logger=None,codec="mpeg4",preset="slow",ffmpeg_params=['-b:v','10000k'])

This works very well even with preset set to slow. This setting gives better quality for the same number of bits and if this is not an issue, you could set it to medium or fast to gain some more on speed.

Answer 2

I realize that this is old but for anyone still having this issue be sure to add progress_bar = False to your code. EG.

arrayClip.write_videofile(output_movie, threads=4, audio = False, progress_bar = False) 

Having the progress bar printing out each time it updates into IDLE takes up a ton of memory, thus slowing down your program until it stops completely.