On their way up, yes.
As far as I know, the warhead is in free fall on the way down, and it decelerates very quickly after launch phase and once it strikes the atmosphere.
I would be really surprised to learn that it detonates at faster than 1,500 mph.
Anyway, I spent 10 minutes on Google and came up with lots of information on launch speed and reentry speed, but nothing on impact speed.
I saw several “terminal velocity” numbers, but those look exactly like the reentry speeds, so I'm dubious.
“I would be really surprised to learn that it detonates at faster than 1,500 mph.”
I’ve tracked countless reentry vehicles. They are far going faster than 1500mph on splashdown.