Bluetooth introduces a delay which the app can't always account for (and which is different depending on the device). In our testing, connecting to a BT speaker can actually cause the playback to become stuttery or choppy. This was with a Moto X, however depending on your phone, your experience may vary.
SOLUTION: If your playback becomes choppy: quit the app completely, disable Bluetooth (or disconnect your speaker) and relaunch the app. If the problem persists, try restarting your phone.