It is very fast. Be aware that it is announced at both sides that you are about to enter the tunnel. This may or may not make a difference to you: I know it does to some.

Also, it may help to get a train that stops at Calais-Fréthun on London-Paris journeys, or Ashford on Paris-London. Although you hopefully don't have to get off there, it can help just to know that the possibility is there to do so, immediately after the tunnel.