Are you using the package of motion that supports MMAL? According to this thread titled: Motion/MMAL/Jessie it sounds like the Pi just can't handle the frame rate.
Your resolution question is a bit more complicated. The clever part of this version of motion is that the Pi's camera feeds low resolution video to the motion detection algorithm - so that the Pi's processor isn't too heavily loaded - and at the same time, higher resolution stills and videos of the detected movement can be saved. So you need to have a good look at your config file to get the resolution you want. These are the key lines in the config
The example config shows the framerate of 4:
NOTE: The distro used above is Raspian.
Further in that thread this comment is also key:
You should be getting the video at 640x480 and the stills at 2560x1920.
There's another link to how to set up MMAL - Re: Motion - MMAL camera.