That could be one of 2 things, either video cache, or hardware decode.
Enabling video caching will store a local compressed and heavily artifacted version of the video you're shooting. There is some speculation that disabling it can improve the fpv performance and make the video feed less prone to dropping.
Enabling hardware decoding is on your apple device (android has it enabled by default), and tells the device to use the GPU to decode video instead of relying on software alone for the decoding. It improves performance and tends to keep the device cooler (since the CPU alone is not doing all the heavy lifting).