I cant comment for the hesitation issue, but my guess for the car not getting up to the correct temperature would be the main thermostat. Does it not get there at all? If it was an inline thermostat at fault I expect it would get to 90 eventually, but not for a while. If it simply isn't getting to temp i'd say main stat. The gauge itself is usually eager, but eventually the real temperature does catch up with the gauge and both will sit around 90.
If your coolant is only 75 degrees then your engine isn't running how it was designed to run, and MPG will suffer because of it.