Indeed the jetstream is a little backwards at the moment ... and has been for a little while now. European inbound flights to the East coast have been generally arriving early (key word: generally!).
For example, the scheduled programme for BOS
is 6h45. The flying time for a while has been 06h30, which does not leave much room for taxi time. So even if the plane leaves on time and has a 20 minute taxi, he's already going to be arriving back in Europe a little late. The average flying time for BOS
is approximately 5h55-06h05.