He, being slow on his feet, had of course already fallen 2 meters behind.
Literal
Feet [subject-が] slow he [topic-は], of-course already 2m gap [subject-が] attached.
足が遅い (slow-footed) is a common expression placed as a relative clause modifying 彼. 差がつく (a gap opens up) is a set phrase for competitive contexts. もちろん (of course) and すでに (already) together convey inevitability. The sentence reads like sports commentary or narrative.