If it makes any difference I was having a similar issue with the galaxy s7 edge. After pairing the watch to the app, I actually paired it a second time to the phone's bluetooth and added it to the collected list of paired devices (it didn't show up on that list with just pairing it to the app). I don't know if the iphone handles devices the same, but I went and added the watch to my trusted devices, which then I think just keeps the device always connected (as long as there is a bluetooth connection. Meaning if I were to turn off BT on the watch it wouldn't see be connected and vice-versa).
There is a BT helper option in the GC app (at least on Android) this may help with transfers, or speed them up.
