When I get deeper into the game I usually build a few super light cruisers with light defensive and offensive capabilities.  I also put on like 3 or 4 engine blocks.  This gives the ships a speed of usually over 12 per turn.  I use them to strike undefended enemy resources far behind the lines.

Problem is, at a certain point the additional engines do not contribute to the ships speed anymore and it will only allow me to move 2 or 3 per turn.

Is this a glitch? or some dynamic of the game that I am not taking into consideration?