It's not dependent solely on speed. If the missile is heading head on towards where the target is, according to physics it will still be able to intercept. It simply means that the area defense coverage of the interceptor is decreased laterally as the hypersonic missile speed increases, so area defense becomes harder. Now it's much harder to tell how hard intercepting a maneuvering warhead would be, but a pure ballistic trajectory should be quite easy for SM-6, or any sufficiently advanced SAM to intercept, due to the fact that the warhead is travelling straight towards the location the interceptor is being launched from.