You do need a reasonably powerful phone (min 2gb RAM)However you can help it out by making sure nothing else is running, try a reboot just before connecting up.
Also your USB cable wants to be good quality, the one they supplied is nothing special. A short one from leading brand is worth considering.
Typically it will start off OK but gradually starts to breakup, in which case you will need to turn off I think they call it on board cache where the phone saves what you are seeing on the screen for speedy review.What's recorded on drone SD card is still full res.