The limit of the X5 for periodic imaging is 5 seconds. We were trying to cheat it before but too many users were having issues with it. DJI says it images at 1 image per 5 seconds and that is what it now does.
We tried to jump through all sorts of hoops before since there weren't any other options but with the X5S and Inspire 2 supporting imaging down to 2 seconds and higher resolution it is the option that should be used if you need to fly fast.
We will not be offering faster imaging with the X5 since it would exceed DJI's published limits.