FWIW, I flew the JFK-YVR r/t sector twice recently -- once in mid-January and once in late March/early April. I had been monitoring on-time stats for both trips in the days prior to my own departure, on the CX website, including the incoming HKG-YVR flights. I definitely noticed that the flights were almost always late in January, but were almost always ontime or early in March. My actual experience with the flights mirrored that -- late both ways in January, ontime or early in March/April.