So I've been working on a spaceplane using my JATO boosters (that use Interceptor missiles), and I've noticed at very high altitudes they accelerate ridiculously fast until the speed suddenly caps to the one in the image below. Is this the top speed of the missile? 
@JacobHardy64 Just tried it, it caps at the same speed. I'm using high physics settings.
try only putting a small cockpit on the front of the JATO and some small wings, and see how fast it goes. What physics settings?