Wow. Somehow I totally missed your extensive post earlier.

The reason I came up with that 20° differential is that that's what the AC service company told me over the phone, and I'm disinclined to believe that they'd lie in order to prevent making money.

I've not been opening windows at night. For one thing, it's not been getting cool enough for that to be helpful, but that's a good point about overworking the AC just to dry out the air.

I know that the AC had the same problem last year. It seems no worse this year, which leads me to believe that it's not a leak. I want to say that for a few years before that, we had relatively mild summers, so I may not have noticed. In addition, I didn't really notice last year. I only really noticed it this year because I happened to be home on a 95°+ day.

75° isn't terrible, but the AC was running all day long. It didn't occur to me that if I'd turned the thermostat up to 75° that it probably would have started cycling normally. I was initially concerned that if it couldn't keep up at 95°, it must have lousy efficiency at 90°, and I was concerned about wasting energy and money.

But when I called the AC service company, they told me that what I was seeing was expected behavior. They said that ACs are usually sized to provide 20° of cooling. But I don't think I've experienced this at other places I've lived.

I'll check to see if the evaporator is frozen. I should also check to see the temperature of the air coming out of the vents.