How do I speedup encoding of a video that has just 1 static frame with ffmpeg and lib264 library?
Give this a try:

ffmpeg -i audio.m4a -loop 1 -r 2 -i image.jpg -c:a copy -c:v libx264 -preset ultrafast -shortest out.mp4


-r 2 sets the frame rate to 2 fps (some players seem to have problems with 1 fps).