Ac performance

I had a 1.5 ton system installed last year which never seemed to perform that well. Today I turned it on at 8:45 AM and set it to 60 and these were temps

8:45 AM
69.8 degrees inside
55% humidity inside
68 degrees outside

11:36 AM
68 degrees inside
52% humidity inside
73.2 degrees outside

2:53 PM
70 degrees inside
52% humidity
83 degrees outside

5:15 PM (house gets shaded out by 4 or so)
72 degrees inside
51% humidity
80.6 degrees outside

I know the fan is set to high, but there doesn't seem to be enough flow to maintain a temp. I removed the filter and flow didn't improve. I would say probably the in/out delta is 15 degrees. There just doesn't seem to be enough of it. This is in VT. It's a 900 sq ft house with walk in basement so 2 floors of cooling. The basement is naturally cool. What it seemed to be doing was exchanging the basement air with 1st floor air because at the end of the day, the basement was warmer that usual.

As I said it never really worked that well and the response from my first complaints early on was that it was working correctly for it's design temp.

I'm thinking it might be undersized a bit.

any ideas?

thanks

Ac performance