There's a limit, yes, but it's not that one. If the land unit has used 80% of whatever its land movement is when it boards the ship, then the ship can move a max of 20% of its naval movement afterwards. If the ship uses 90% of its move the next turn getting to shore, then the army only has 10% of its movement left. I'm not referring to the turn the trebuchets load, I'm talking about the following turns. I had more troops than would fit on one fleet, as I was dragging along militia as a garrison too. The ships with the trebuchets were slower than the ships without.