Whatshername is incorrect about everything she said, except the cleaning. Cleaning the outdoor unit with a garden hose at medium spray will make a difference. Unscrew the top and turn it to the side. First, turn the POWER at the disconnect OFF. You don't want that fan blade going. Turn the tstat to off, too. Then, clean all of the dirt and leaves out of the bottom. Take the hose and spray inside to out, for twenty minutes. You will get wet! Then, put the top back and turn the power back on.
Closing a vent increases the static air pressure in the ducts, effects the air flow by creating turbulence and makes the blower motor work harder against that increased pressure.
Also, you are just wasting cold air in that duct that you're paying for. And, that hot room air from the closed room is drawn out into the return to mix with the rest of the house air and raises the temp indoors. You can close the door, but there's no extra cooling effect. Anyway, I know cleaning the condenser will help. I've done hundreds.
The evaporator fins can be cleaned to, but they're difficult to get to, but you might try that, too.