who would win at chess boxing Magnus Carlson or superman

Superman would win because he'd knock Carlsen unconscious and Carlsen would then be unable to play chess and would lose on time.
Round 1 is playing chess. Magnus could use his Bullet Chess mode and makes fast moves.
It would be inadequate. The referee will nudge a player after a 20 second think, disqualify them after 30-40 seconds. Superman could easily stall, make nine moves of 20 seconds each and get a little behind on his clock, but avoid being checkmated.
Round 2 is boxing. Magnus Carlsen couldn't last 3 minutes against the Man of Steel. There would be no round 3.