[burfog]

Incoming speed doesn't help the attacker unless it beats the sensors, which operate at the speed of light. Mach 3 missiles impact Mach 0.1 shrapnel at about Mach 3.

That helps the defender. The faster the missile goes, the harder it gets hit by defensive shrapnel.

[greedo]

Speed makes the engagement cycle short; instead of having maybe 2 shots at an incoming missile, you get one. And if your system is too slow, it has no chance of reacting in time. Most systems are designed to defend against missiles, but are ineffective against cannon rounds (APFDS).