Is there a hard speed limit to these things, or is it different for all the ships?

It's different for all of the ships, and this is mostly for gameplay balance, because say, for example, you have an interceptor ship that's designed to catch up with other ships. It obviously needs to have a higher speed limit so that it can catch up to things. And we justify this as certain ships are better at coping with higher speeds and are better able to control their accelerations at higher speeds, and so they get higher speeds.