Actually with Scoot to Gate, as long you reach airport like 40 mins before scheduled departure time, you wouldn't miss your flight.Scoot-to-Gate is the game changer for me.
Of course if kana those unforseen Singapore Immigration computer system fucked up for a few hours type rare disaster which happened some time ago, it's generally safe to arrive at airport 40mins before departure and chiong to immigration and security clearance then chiong to boarding gate.