The LGA route is the most aggressive; 670 leaves DEN at 11:10a, while 1336 leaves at 4:25p. The reverse flights are at 8:10a (1675) and 1247 (1:25p). On Saturdays only one nonstop exists: 531 leaving DEN at 5:35p and 849 leaving LGA at 2:35p. It looks like all flights could be made by an aircraft doing LGA-DEN-LGA, though the weekday flights would require two aircraft.

This route otherwise has seven daily flights on it (except Sat), three from Frontier (2x320, 1x319) and five from United (2x320, 2x319, 1x752). On Saturday, the schedule thins by a flight or two, not including FL's drop from two to one. Pricing on this route is quite competitive: ~$130 OW is the floor I've seen.

By way of comparison, JFK has two flights per day (one Delta 738, one JetBlue 320) and EWR has nine (3x WN, 6x UA/CO, specifically 3x738, 1x320, 1x753, 1x739). So, between JFK, EWR and LGA, WN/FL will have five flights to UA/CO's 11, though on average WN's planes carry less passengers.

Is WN trying to push out Frontier on this route as well? Maybe, though the impact of adding ~275 seats on Sun-Fri to LGA isn't nearly as significant as adding 40% of that in a much smaller market like CAK.