What does it matter anyway?
So long as you get there in one piece ~ what is this obsession with getting there faster?
The scenery is pleasant, in my experience the food on Eurostar is excellent and the wine is good.
Lunch eaten on route to/from Paris is as good a way of spending three hours as I can imagine and if it takes four hours there's time for a post prandial nap as well.
What is the point of rushing to get to London and then have to spend hours getting anywhere from there?