A/C not keeping up

So, the first warm-ish day of summer finally showed up in the Black Hills yesterday and I noticed that our central air conditioning was not really keeping up… the thermostat was set at 72, but the interior temperature was 76 and didn't drop a degree after several hours of continuous operation of the A/C.

Troubleshooting, I identified the following issues:

• only two open/unblocked registers in the entire house. Solution: opened/unblocked all registers and delivered a semi-patronizing lecture to my children about not covering the vents with boxes.
• An extremely dirty air filter. Solution: changed air filter. Stern lecture to myself about remembering to do this every 60 days or so.

Thinking that this would solve the problem, I sat on the couch and waited to be bathed in the A/C's cool embrace. Yet, after another hour, the temperature had still not dropped even a degree. The next steps in my investigation revealed the following:

• the air blowing out of the registers didn't seem all that cool (but it did seem slightly cooler than room temp).
• the insulation covering the line coming in from the outside unit were soaking wet and had visible condensation.
• the outside unit seemed to be blowing cold air.

I've taken a look at this question:

AC Unit Blow Warm Air Indoor But Colder Air Outdoor

but I am daring to risk being flagged as a duplicate due to the additional details I have provided, hoping that they might help in the further diagnosis of my problem. Is there anything else I can try or do I need to call upon the arcane services of an HVAC professional?