He's very inefficient.
He extends and contracts his right elbow joint with each stride (not so much on the left), which destabilises his torso and results in that pronounced head movement from side to side. It's why he looks like he's swimming through treacle in the last 300m.
Most runners have a slight deviation of the elbow angle through the entire gait cycle, but that tends to be as a consequences of the jolt in each stride. Holt is actively contracting a muscle he doesn't need to contract.
He'd be much faster (especially at the end of races) if he worked on this.
But he's still faster than me, so what the hell do I know...