We do learn that for $\mathrm{S_N1}$ reaction the rates are tertiary > secondary > primary > methyl. The reasons mentioned are usually inductive and hyper-conjugative effect.
But which of the effects is more dominant? So, let's take two chlorides, ethyl chloride and isobutyl chloride, which will be more reactive to $\mathrm{S_N1}$? If we refer to inductive effect, then isobutyl chloride should be more reactive. But if we refer to hyper-conjugation, then ethyl chloride should be more reactive.
Which is more reactive actually?
Note: Which carbocation is more stable, the ethyl- or 1-propyl-carbocation? is not the same as this as I am not satisfied with the answer there. That question compares the stability of carbocation while I am comparing the reaction rates which may depend on kinetic factors too. Actual data could be helpful.
Also, the isobutyl cation rearranges to a tertiary cation. Does the stability of rearranged carbocation affect the rate of reactivity?