So couple I points
1. Yes it has increased your average frame rate has increased from 233fps to 263fps with -P which is like a 15% increase.
That's pretty good (not awesome) considering that there is no cropping and scaling happening. If you scale and crop your videos then you should see an even greater increase. You try it by reducing the max width to say 320 and then run with and without
-P and see the difference in the average fps
2. The amount of fps depends upon the hardware and driver implementation
3. There is another option -U. Try adding that in addition to -P and see if that helps further. It offloads decoding to OpenCL hardware. This helps particicularly with slower CPUs