Yes. It wouldn't be possible in some of these missions to get these times, even if you could run all the way with no enemies to fight.
There are definitely hackers. One advertises his hacks and sells them, so there's a lot of people using them. I've reported the user, sent videos of him doing it, his Ubisoft account name, etc., but they aren't interested in stopping him. They respond with "We can't disclose any actions against the reported player", but I know they've done nothing because his account is still active.
He has a double-barrel shotgun that never needs to reload. It fires at around 2000 rpm and shoots through any wall or obstacle. Unless this is some new Exotic I don't know about, I'd say it's hacking.