Central air conditioning does not work well.?

My wife and I just bought a duplex with central air in both sides (seperate units). Unfortunately our AC seems to not work well. It was 78 degrees in the house, about 77 degrees outside. We set the AC to cool to 75, and it took about 45 minutes. Sounds like a mive waste of electricity to me. Since I have installed AC units before (with my father) I know a little about them. I changed the filter and checked the registers. Only cool air is blowing out when it comes on. I'm hoping its not a freon issue, since I remember charging units, but I cant remember which of the three hoses on the gauges go to the unit and which one goes to the tank. Any suggestions/thoughts? Thanks!