My split A/C unit used to drain much water when it was new. Now, the unit drains much less water than before. I checked the drain line and it isn't clogged. Also, the cooling efficiency is slightly lower than before.
You need to find out why the cooling efficiency is slightly lower. Offhand, I'd guess it's a clogged filter.
If it's low on gas, it will freeze up the coil and produce less water (and not work very well), but a whole bunch should come out when it shuts off and melts This should happen within a few minutes, so don't walk away, while you:
Let it run for a good while, and then set the fan to
on and the A/C to
off; what happens? If lots of water starts coming out, you're low on gas and your coil is freezing up.
You can confirm this by removing the panel from the coil, and you'll see it's all froze up. Seeing frost on the 'fat' line (at just outside of where it enters the coil) is a good indicator, too.
I agree with @Maura but low air flow from plugged filters or a dirty coil can cause the same thing. Yesterday I replaced a small unit that was not cooling at work, the filter was clean. I decided to remove the covers and found the evaporator coil to be totally plugged (inside coil) cleaned it with coil cleaner and is is now fully functional. The outside condenser coil if dirty will also reduce how much heat the unit can exchange if it is plugged with dust it may not be able to remove enough heat. A simple garden hose used to flush the outside may also get you back to like new.