Well thank you for playing!
I think why it seemed to you that it didn’t respond is that the speed is accumulative (for realistic purposes I guess? I’m pretty sure Kerbal Space Program does it the same way), so when you already going to a certain direction and fire up to another direction, the original speed still remains, just added by the new heading and momentum. Another variable that can messed up your speed is the gravity of the planets and the sun, so that could also be a thing (or possibly the fuel ran out. if that’s the case, possibly I have to fix my ui design to show it more clearly).
Or maybe you encountered a bug that I didn’t find or a bug that is right in front of me and I’m blind at looking for it. Anyways, thank you for the comment and possibly a bug report?