1. Efficiency has nothing to do with how long the unit runs. Your 25 year old system had a low seer and your new one 10 seer plus.
2. Set the thermostat and forget it (was also good advice from another poster) IF you want to screw with the thermostat get a programmable one and set back a few degrees.
3. A properly sized system will not pull down temperature very fast on the hottest day. Qac - Qload = "thermal mass" X dT/dt At the hottest part of the year the capacity of the ac just meets the load therfore it won't be moving temperature all that quick. "set it and forget it" you wont save any money by shutting off the unit because when you turn it back on not only are you cooling the temperature of the air in the house but you also have to remove the heat on the "thermal mass" or in other words the heat contained in the building envelope and all the contents.
4. Did you go with the lowest bidder? If you did then dont' be surprized if they dont have a micron guage and dont pull a vacuum but purge the system. Get the system tuned by a reputable company to check the charge.