[FFmpeg-user] Speed up overlaying process

[Dzung Nguyen]

I'm overlaying two videos together:

ffmpeg -i video1.mpg -i video2.mpg -filter_complex
"color=c=black:size=1280x720[base]; [0:v] setpts=PTS_STARTPTS,
scale=856x642 [left]; [1:v] setpts=PTS_STARTPTS, scale=424x318
[right]; [base][left] overlay=shortest=1:y=39[tmp1]; [tmp1][right]
overlay=shortest=1:x=856:y=201" -q:v 0 output.mpg

This takes very long time, for a final output of 1hour. Which setting
should I change to make it faster? (remove scale filter)?