Steric hindrance in Nucleophilic Substitution

CH3-X          very fast SN2 reaction
RCH2-X        fast SN2 reaction
R2CH-X        slow SN2 reaction
R3C-X           SN1

The steric hindrance slow down the SN2 rate.
* If there are two steps in a single mechanism,  the slower of the two determines the rate of the overall reaction.
* If there are two different mechanisms avaiable under the reaction conditions, only the faster of the two actually occurs.

On molecules with very few steric hindrance (or any of it) the SN2 reaction mechanism is so fast that SN1 does not get a chance.