Hmmm where does it say that?
I know that allylic and benzylic halides react faster in SN1 because the rate-limiting step is the formation of the carbocation, and allylic and benzylic carbocations are more stable than even tertiary carbocations.
But since no carbocation is formed in SN2 and its mechanism is concerted, I thought that steric hindranced would be more important than all that. Can anyone else confirm this?