I have seen a perfectly good roof air replaced because the evaporator was plugged with gunk. Could not get enough air through it.
The evaporator is the coil you see when you look up into the AC from inside.
Then need to check Delta T. “Delta T” is the most common use of the word delta in the HVAC industry, meaning temperature difference. If the temperature before a cooling coil is 75F and the temperature after the cooling coil is 55F, subtract 55F from a 75F to find a delta t of 20F.
I would do a couple Googles on how to check efficiency of AC. No sense in adding a 2nd unit if that one is going bad or replacing a good unit.