Your current description sounds about right. In the campaigns, Super Abilities appear to be disabled, meaning the speed reduction wouldn't exist (I assume you're not playing on the original Dread Lords, in which the speed reduction is absent because DL had no Super Abilities). I'm not sure what could have caused the earlier result (speed reduced by one) assuming you're remembering it correctly.

This can be caused by Military Starbases, who can have 2 modules attached to it that nerf -1 and -2 movepoints. In Yorspace very effective, as it can reduced enemy ships to 0 permanently.

It could also be caused by passing asteroids.

Then, I've encountered alot of issues with move-points when entering a new turn. Esp. when a lot of ships are present, the game behaves pretty erratic. Examples are... start a new turn but some ships/fleets don't get their movepoints replenished, and have to pause this turn... ship crossed an asteroid field but still has to move with 1 speed although its already away from the asteroids for 2 or 3 turns.... ships enters an asteroid field, but in the next turn has still its full movepoints, so I can fly it out and circumvent the asteroids.

Too much stuff to wonder about if you ask me.