They slow down to much slower velocity on re-entry.
From Mach 18 at Reentry at 60km altitude, it will decelerate to may be Mach 6 at hitting the target or detonation,which almost always happens at 1000 meters or above ground, in case of nuclear.
Even Mach 6 is classed as hypersonic.
Both the missile reentry speed and subsequent hitting the target speed, depends on launch profile and the resultant trajectory.
At lofted trajectory the missile loses some range , and takes slightly longer to hit the target. But as the name suggests, it's lofted and dives towards earth from much higher apogee, it gains massive speeds.
Minuteman used to re-enter at speeds of way over Mach 23 . With an apogee of 1300 km.
Pakistani Shaheen 3 was tested with apogee of 550 km. Reaches Mach 18
These high altitudes give the re entry vehicle enough time in the vacuum of space to free fall under gravity and add to the speed, initially gained by rocket motors.
The crazy North Koreans send their missiles to almost the moon.
Heading 18 was recorded at an altitude of 6600 km before diving down towards the earth. Reaching silly speeds , around Mach 30 at re-entry.
Atmosphere is only 60 km thick. Warheads entering at such high speeds still remain hypersonic when hitting target or detonation,, after decelerating to a third of initial speed.
