Interesting -- AA had more weather delays but still had a much better on-time performances than UA. Took 5 carriers and compared them.

Ontime
UA - 70.11% (5)
US - 86.16% (1)
DL - 85.49% (2)
AA - 79.81% (3)
WN - 79.85% (4)

Causes of Delay
Air Carrier Delay
UA - 9.70% (5)
US - 4.08% (1)
DL - 4.54% (2)
AA - 5.82% (3)
WN - 6.23% (4)

Extreme Weather Delay
UA - 0.49% (4)
US - 0.24% (1)
DL - 0.41% (3)
AA - 1.28% (5)
WN - 0.36% (2)

National Aviation System Delay
UA - 8.26% (5)
US - 5.56% (3)
DL - 4.67% (2)
AA - 5.65% (4)
WN - 3.00% (1)

Security Delay
UA - 0.06% (3)
US - 0.08% (4)
DL - 0.00% (1)
AA - 0.04% (2)
WN - 0.08% (4)

Late Arriving Aircraft Delay
UA - 9.47% (4)
US - 2.88% (1)
DL - 4.21% (2)
AA - 5.43% (3)
WN - 9.62% (5)