If the separation of halide ion from carbon is the rate-determining step in the SN1 reaction, one would want a solvent which stabilizes the resulting cation and anion to speed up the reaction. Which solvent would you choose for this
role? (Hint: Bruice, 5thEdn., Table 8.7; Bruice, 4thEdn., Table 10.7.
This solution explains what solvent type stabilizes Sn1 reactions.