F-18 is only mach 1.8 and can only pass the sound barrier with afterburners
the f-22 was designed with the idea of supercruise, ie. being able to reach speed of sound with just military power (ie no afterburners) so it uses less fuel and can stay faster longer than f-16's, 15's, 18's etc. So a mach speed of 1.5-1.8 for long periods is better than short jumps with todays more conventional aircraft.