$ time ffmpeg -v quiet -f lavfi -i testsrc=s=1280x720 -c:v libx264 -preset veryslow -crf 15 -frames:v 1000 -f null -

real    0m19.422s
user    0m0.015s
sys     0m0.000s

$ time ffmpeg -v quiet -f lavfi -i testsrc=s=1280x720 -c:v libx264 -preset veryslow -crf 25 -frames:v 1000 -f null -

real    0m18.374s
user    0m0.000s
sys     0m0.000s