When tert-butyl bromide undergoes SN1 hydrolysis, adding a common ion (e.g., NaBr) to the aqueous solution has

Question:

When tert-butyl bromide undergoes SN1 hydrolysis, adding a "common ion" (e.g., NaBr) to the aqueous solution has no effect on the rate. On the other hand, when (C6H5)2CHBr undergoes SN1 hydrolysis, adding NaBr retards the reaction. Given that the (C6H5)2CH+ cation is known to be much more stable than the (CH3)3C+ cation, provide an explanation for the different behavior of the two compounds.