Hello, new to the forum!
I have an iPhone 5 and the P3S
I had the very same issue you mentioned at first, very short range with the 1.6.8, and a slow app response with the 1.5.7. Between the two, I chose the 1.5.7, as it allowed for my to fly further. I don't know if this will work for you, but I deleted and reinstalled the app on my phone, deleting as well all the cache, etc. Then , I reopened it, connected to my drone, and the problem persisted. Without switching off the drone nor the RC, I exited the app and relaunched again, and it worked! I made a test flight afterwards, and didn't have any lag issues! I have had the drone for a few time, therefore I have not had the opportunity to test the max flight distance, but I do not have the range issues I had with the 1.6.8