You'll only need GPS on your mobile device if you want to use the Follow flight mode or if you're going to need to reset the home point to your current location after taking off. All other GPS related features use the GPS receiver located inside of the X-Star.
As for devices, the Nvidia Shield K1 is a great Android device. For Apple devices, an iPad Pro, iPad Mini 2+, or iPad Air 2 are good options. A lot of Android devices contain GPS receivers. Only the cellular version of Apple devices contain GPS receivers.
Yeah, I'm looking at this today! Nice price drop on the 8gb 2016 model, (119.00 at Best Buy OR $100 for "Excellent Open Box")..not bad at all. I do have an older 2012 iPad Mini but noticed a bad lag on the app.I use the Samsung Galaxy Tab A 2016 and works flawless. Under $300.00, make sure it is the 2016 model, older ones are not fast enough.
Not sure if this is related to your problem but ever since I updated to the beta firmware I have started to get pixelation or lag on my screen in places I never used to get it. It is not real bad and it is not all the time but it seems to have developed ever since the new firmware.