If you look at flight tracking websites you can often find groundspeed. Compare that for a selection of westbound and eastbound flights. You will soon get an idea why westbound flights take longer.
Incidently Cathay put a lot of detail on Flightstats (perhaps too much, including several changes in estimated arrival time) but you can get a lot of useful information.
It is interesting to look at old timetables and see how the timetabled block time has changed throughout the years. It has increased significantly for short haul flights to and from Heathrow over the last twenty years owing to longer taxi times and more time in the stack. The number of movements at LHR has increased significantly over the period but at a cost - ATC will land several widebodies followed by several narrow bodies which increased capacity owing the increased separation required when the later follows the former.
There are instances where an airline will timetable different block times on different days of the week owing to different routings - it may fly over certain countries in order to retain overflying rights. (Have a look at EK 125 from DXB - VIE which is 35 minutes quicker on Thursday.)
I booked a flight between JFK & DCA a few years ago (DL 3115 operated by Mesaba) which was timetabled to take 1 hr 55 mins. It covered the 215 miles in about a third of this time. However you can often be delayed out of JFK and when I checked the statistics the flight reported very good punctuality!