Not necessarily timer coalescing and probably more of a radio optimization. All 3rd party code will run on the phone and not the watch, so any updates to the UI

Not necessarily timer coalescing and probably more of a radio optimization. All 3rd party code will run on the phone and not the watch, so any updates to the UI made via code will have to necessarily travel over Bluetooth. Apple wants to limit the frequency of these updates, so the most frequently updated UI bits (clocks, timers) will be done on the device itself without the persistent BT use.