HS2 is not so much about speed as capacity. It does not replace the existing lines, it adds to them. That's why it's been so controversial - it puts railway where there was none. It turns Coventry and Birmingham into London commuter suburbs.
Capacity is only needed for short periods at peak times, which could be achieved by e.g running more standard-class carriages instead of first class. [0]
Since you mentioned Coventry, it's intercity capacity to London will actually be reduced if HS2 comes to be due to extra trains to Birmingham occupying the track. Same for many other towns and cities in the Midlands. [1]
