What would be the effect of carrying out the sodium iodide in acetone reaction with the alkyhalides using an iodide solution half as concentrated?

A reaction with alkyl halides in NaI with acetone is by the Sn2 mechanism. The rate for an Sn2 mechanism is directly proportional to the concentration of the nucleophile:

rate = k[nucleophile][alkylhalide]

If the iodine solution (the nucleophile) is half as concentrated, then the rate will also be halved.

rate = k [nucleophile]/2 [alkyl halide]