Not necessarily. The window to act to make a turnover is so short that speed on foot isn't going to make much of a difference, because if you have to travel any kind of distance on foot, you're not going to get there in time anyway. Most turnovers are done either by the tackler rising back to his feet, or the first person into a ruck. It's much more important to have speed to and from the ground and/or strength to drive over the ball in this case than pace. Although I suppose you could include the former in being slow. Robshaw certainly suffers from not being as fast up on his feet after a tackle compared to the better 7s.
I disagree. When I think about a traditional/classic openside, I think of someone with pace, who is adept in both the tight and the loose. They are the ones offering support in attack, to either continue the move, or protect the ball at the ensuring ruck when the rest of the pack will still be trying to catch up. Some might argue that one of the other backline players can do this job, they're big enough. However, having the rest of the backline players in position for the next wave of attack is important, so tying them into rucks will just limit the next phase.
Imo, Tipuric is almost the perfect traditional 7. He's physical enough to get stuck into the general role of a forward, carrying the ball, defending the fringes; but he's also quick enough and skilful enough to play as an extra backline player. Take the first try he scored against the Dragons last week, he was able to support Matthew Morgan when 95% of forwards would be left tens of metres behind, and was there to take a pop pass from the ground. If the ball hadn't of come, he would also have been there to protect the ball in the ruck and provide quick ball on the next phase to the O's backs could finish the job against a disorganised defence.
So imho, pace is what separates what I call a traditional openside from a blindside type of player playing at 7.
Edit. Being a good jackler is a more modern role for an openside, but it stems from this combination of physicality and pace, making the openside the outstanding candidate for the job in defensive situations. Whilst someone like Dan Cole is an excellent jackler, he doesn't necessarily have the pace over the first few metres to get himself into as many rucks as someone like Pocock or Warburton. This is where Robshaw struggles slightly as well, when someone is tackled right in front of him, he can get over the ball and is physical enough to stay there, but if the tackled player is a few metres away, he doesn't have the pace to get there before the opposition backrow, and thus struggles to get himself into a good position.
It was evident a couple of years ago with Warburton, that after a series of injuries, he'd lost half a yard of pace, and struggled to make much impact at the breakdown.