This can be caused by two different things: the compatibility of the SD card with your camera and the CPU load on your iOS device.
If you can't use the intervalometer on the DJI app at the same speed you have the writing set to in the SD Card Speed setting in Map Pilot it isn't going to work. The DJI intervalometer can be set to take an image every 2 seconds but lots of cards don't actually work with that. You have to test your card to verify this.
If your iOS device is older than an A10 processor you might need to turn off some features to free up the CPU/GPU. Make sure the Show Image Footprint setting is turned off and that you have enabled Auto Clear Image Points. These are the two biggest offenders. Beyond those, make sure other apps, especially other drones related apps, are fully closed and not running in the background.