The great circle distance between the two stations is only 108km, so I'd hope (but not expect, sadly!) it would be substantially faster than that. In order to compete with the by-then much more completed Oxford-Cambridge Expressway, even more so. It's 2h door to door from my house in Oxford to my friend's in Cambridge, even in rush hour traffic, using the current road network. Another aspect of its viability is that as well as the speed being sufficient, the fares will have to be a lot cheaper than the current options. Perhaps it will indeed end up being more of a “metro” service for the various towns and cities along its route,
As for Oxford <-> Cambridge, at the moment it's £46.85 for an off-peak return (using a Network Card), taking 2h30m station-to-station (3h8m door-to-door). That's more than a full tank of petrol, which would do the trip twice with some left in the tank.
Both these aspects need to be competitive to make any sense at all, especially when compared to a by-then normal electric car on the by-then more complete expressway. Whatever or whoever one blames for this (hidden subsidies etc.), it doesn't look great. 90 minutes and £30 off-peak return would make it look marginal to me … 60 minutes and £20 and I'd use the train every time. Let's see how it all pans out.