The data rate of Bluetooth 5 is theoretically doubled from 1 mbps to 2 mbps, versus version 4.2. The range is also much further. However, my understanding is that you cannot get both long range and high data rate. If the phone and watch are somewhat close to each other, the data transfer speed could be double, but this assumes Apple will not throttle the connection to save battery consumption on either device. Guess we'll see once Series 4 shows up in the wild. Apple did not seem to highlight that the Series 4 has BT 5, so makes me think we aren't going to see a huge improvement.