No, the main issue for that route is definitely 125th Street. LGA can sometimes be bad under normal circumstances, but work is currently being done over there, making the current situation anything but. (If it can't get off 125th Street in a timely fashion going to Queens, it'll get to LGA late, thereby making the return run late, and that has a domino effect, especially since it's not just one or two trips.
Hell, the traffic on that one street was cited in that Bx15/M125 split proposal...