Superchargers spin "at speeds related to that of the crankshaft", they are geared (quite complex) to suit their application eg... flow rate bearing in mind crankshaft speeds etc.
A supercharger generally spins faster than 12k-15k rpm more like up to 50k rpm. But yes you are correct in thinking they dont come anywhere near to that of a turbochargers.
A supercharger doesnt use the same oil as that of the engine so isnt really subjected to the same issues related to "letting it cool down".
no special shutdown procedure is required with superchargers. Because they are not lubricated by engine oil, they can be shut down normally.
http://auto.howstuffworks.com/supercharger5.htmBut as mentioned before turbochargers obtain their power from exhuast gases flowing through a turbine, which especially after a good thrashing can tend to glow red with heat. When shutting down the engine there will be no passing airflow through the turbine, the blades/spindle/bearing (sleeve or ball) will absorb this heat and hey presto f00ked turbo.
Consider then that a supercharger receives it's energy from the crank via a belt/chain mechanism which doesnt subject the unit to nowhere near as much heat.
But as good engineer's always say, you must consider your application. Fact for fact, superchargers tend to prefer large displacement engine's, mostly due to the reason that the cranks supply ample energy - consider the rolls royce merlin AND where space is paramount (BMW Mini cooper etc). But then thats a debate for another topic.