Overnight flights from east coast to EU are late enough that they don't impact productive time, and largely get in early enough for a full day of work.
Making that flight shorter - as anyone who's done a DEN-NYC redeye will tell you - doesn't actually help productivity.
Sure, on the return flight, it would be great to shave a couple hours off, but on the way there, with the time changes required, faster doesn't so much help with productivity. Especially with inflight internet these days.