SN1 mecahnisms are uni-molecular(there is only 1 molecule that is rate determining).SN1 mechanisms normally prefer tertiary or secondary substituted compounds.In SNI there is the formation of a carbocation and is therefore a two step process.Here doubling the Nucleophile has no effect on the rection,but the rate solely depends on the concentration of the substrate.
SN2 mechanism are bi-molecular( Bimolecular reactions are reactions in which two molecules collide in the rate determing (slow) step of the reaction mechanism) and a one step process.Here if the Organic substrate was doubled, this will results in the doubling of the rate of the reaction. In addition, doubling the concentration of the nucleophile used will double the rate as well.
In SNI the fromation of the carbocation is slow therefore,it`s the rate determiming step.
In SN2 since two reacting species are involved and is slow,it`s the rate determining step.