At this short of distance it would be slacking off. However if you watch his world breaking run (vid below) to me it appears he's likely not doing that.
Personally would argue these guys do this same sprint/distance so many times in practice/competition that they generally implicitly know when they're running a WR type time and generally don't slack off (in this video he's not in any threat of being caught but still presses on for the WR).
