[Christ]

Lower initial speed because you're less engine drag from compression braking.

If you do compression brake on a carb'd engine, TURN THE KEY ON. You don't want raw fuel cycling through the cylinders from the idle circuit, it will wash the cylinders and damage the engine over time.

If you aren't constantly tapping your brakes to make sure they're working while driving, why would you be worried about it with the engine off? (I get that it was a joke)

If you've got the transmission in gear and you're coasting engine off with the clutch depressed (disengaged), you're also spinning the input shaft and the clutch plate, which is only a few thou of an inch away from the pressure plate and flywheel. There's still some friction, some heat being generated from it, and it's extra rotating mass.

Leave it in neutral and take your foot off the clutch. It's less drag, and it's not like you can't stomp the pedal, put it in gear and pop the clutch out to save your backside in an OHWHATTHEHOLYMOTHER situation.

Getting the engine MOVING, whether there's ignition or not, will provide drag to create engine braking. Actually, being ON lessens the effect of engine braking slightly because that fuel is still igniting and creating cylinder pressure.

If you want to continue using engine braking, create a throttle bypass that allows air in under the carb and open it any time you want to engine brake. This will prevent vacuum from pulling fuel into the engine so you can leave it off, still be compression braking, and not damaging your engine or burning fuel.