Ismael Tajouri, Maximiliano Moralez, Jo Inge Berget and Ebenezer Ofori scored as NYCFC stayed top of the east with five wins in six matches.
New York was a class above at Yankee Stadium midweek, scoring three goals in the first half, even with Spanish superstar David Villa not in the starting side.
Tajouri opened the scoring in the 12th minute when he finished from the edge of the box with a lovely curled short for his fourth goal of the season.
New York doubled the lead around at half-hour thanks to Moralez, who converted a penalty having been fouled by RSL defender Justen Glad.
Berget opened his account for NYCFC after tucking the ball beyond RSL goalkeeper Nick Rimando four minutes before halftime, to give the side a comfortable lead heading into the break.
The rout was sealed with 20 minutes remaining as Ofori tapped home his first goal for the club, Moralez continuing his fine night with a lovely assist.
The win leaves NYCFC four points clear at the top of the Eastern Conference, though the club has played one more match than Atlanta United, which currently sits second in the table.
NYCFC is also one of just two unbeaten teams remaining in MLS this season, with five wins and a draw from the club's six games. FC Dallas - one win and three draws in four games played - in the only other unbeaten side in the league.