I've only seen spring thermostats.

What engine is that on?

Reason would have it that a thermostat wouldn't open quickly but take a minute or two. You could have an engine starting in sub zero temperatures and if it snaps open when it gets to temperature it would throw sub freezing coolant through the engine. That doesn't sound good.

I'd think they'd open slowly because it's necessary on an engine.

Of course, I'm just musing here.