why is mpeg4 encoder faster than libx264 using ffmpeg

Question

I have been padding mp4 videos so they can become of the size 1920x1080 instead of 1920x800. I used initially used mpeg4 encoder using ffmpeg, the encoding was quick but the size of the file was more than 5 times larger than the original.

I decided to use libx264 encoder instead but the encoding process was much slower than the mpeg4 encoder although the size of the output was close to the original file.

I use -q:v 0 for the best quality, and I am intending to encode using the best possible quality. Is there an encoder that would encode x264 as fast as mpeg4 encoder, or at least faster than libx264 encoder?

Solution

I figured out that if you don't mind a larger file size, you can use -preset ultrafast and the speed will be similar to mpeg4 encoder for libx264 encoder