Does 2-chloro-2-methylpentane react with NaI faster in acetone or water? Does 2-chloro-2-methylpentane react with NaI faster in acetone or water?

I thought that since it is a tertiary halide it will proceed via SN1 reaction. So it must be faster in water as it is protic solvent. 

But the answer given is Acetone. Why?
It  is simple this is the famous finkelstein reaction.


As you can see in the image NaCl is formed which is insoluble in acetone hence it precipitates and the reaction cannot go reverse.


But if we take water the reaction will shift in backward direction as NaCl is soluble in water.

 That is the reason we take acetone.