His speed isn't the issue but it's his first step that to me is the problem. Once he's going he can move, but takes him too long to get there.
But I think it's mitigated playing down low in the middle than on the wing the way the game is played now.
He must of set a record this post-season for taking the puck from his own blue-line along the right-side wall to the red line and dumping the puck in. His first step was so poor that by the time he got to the red line, back-checking pressure was all over him and he had no other play to make. Just killed his lines offense and wasted multiple zone break-outs with these constant dump-ins that typically resulted in loss of puck possession.
However we want to dissect it, overall progress down the ice is too slow. And while you might be right that that's less of an issue defensively if he's supporting the dmen down low, it also means he's not part of the offense as much through the neutral zone and into the offensive zone. Either way, why would this be something we WANT from our 3rd line center?
I guess I'm just saying...yes, maybe he's less of a problem playing center than wing, but if we want to be a Cup contender, shouldn't we aim higher? We need a center who can push David Krejci and help Krejci/Bergeron draw better matchups, not a center who gets pushed by the 4th line center for ice time. Backes just isn't good enough to be that guy, so why force it? Unless, and this may be the case, we are conceding that we are stuck with him, he has to play, we don't have the ability to get a good 3C in the market, we don't have a better 3C in the system, so screw it. If that's where we are at, then sure, go for it.