if the carbocation is stable then how will the reaction proceed faster in SN1 Mechanism?

In an SN1 reaction, the rate determining step is the loss of the leaving group to form the intermediate carbocation. The more stable the carbocation is, the easier it is to form, and the faster the SN1 reaction will be.  Some of we fall into the trap of thinking that the system with the less stable carbocation will react fastest, but they are forgetting that it is the generation of the carbocation that is rate determining. 
Since a carbocation intermediate is formed, there is the possibility of rearrangements (e.g.1,2-hydride or 1,2-alkyl shifts) to generate a more stable carbocation. This is usually indicated by a change in the position of the substituent or a change in the carbon skeleton of the product when compared to the starting material.