Answer
In a $S_{N}1$ reaction, the first step is a slow and rate determining step. It’s the formation of carbocation intermediate.
This carbocation is attacked by the nucleophile much faster. Nucleophilic attack on the carbocation happens almost as soon as it forms.
In case of 3-bromocyclohexene and benzyl bromide, the formed carbocations are stabilized by resonance. Resonance stabilization enhances the rate of $S_{N}1$ reaction.
In case of 3-bromocyclohexene, carbocation formed is stabilized with the positive charge equally distributed on two carbon atoms.
Similarly, in case of benzyl bromide, the carbocation is stabilized by delocalization of positive charge on the benzene ring.
Work Step by Step
Resonance structure for benzyl carbocation is provided in the attachment below
