The words replay in Puka Nacua’s head, again and again: “I … AM … SPEED. “I … AM … SPEED. “I … AM … SPEED.” He borrowed the mantra from Lightning McQueen, the underdog rookie from Cars, and has repeated it to himself before every Los Angeles Rams game this season, convincing himself that he belongs on this NFL stage, despite falling to the fifth round of the draft in part because he ran too slowly in the 40-yard dash.