Tokyo to Nagoya now is about 95-100 minutes: 50 minutes is the time difference, not the current duration. It's a little over twice as fast.

I also imagine that like the (cancelled) HS2 in the UK, a major driver is not only faster journeys (though HS2 was a much more marginal time saving, certainly not half) but relieving capacity on the existing "stopping" line, which has 365 trains a day, 450,000 daily passengers and is the busiest high speed line in the world.