Agree with the above. Main engines control max speed. Engines benefits can add up from 3 things.

Volume = how much space it takes up which controls how much total thrust it provides.

Material = how much it weighs (mass). Lighter material increases your thrust per weight ratio which increases acceleration.

Engineers = at 100% you function as expected. You can add surplus of engineers for up 130% benefit. (You can add twice as many engineers that you need, but the benefit stops at 130%).

This is important to keep in mind for when you add engines. Let's say you have a super huge engine that gives 500 units of thrust. Your at %100 for engineers. You add a little bitty engine, it gives 1 unit of thrust. You entire efficiency goes to 99% because you forgot to hire the extra engineer.

99% of 500 is 495. Plus your little engine running at 99% so another .9 thrust.

Your new Total thrust is 495.9, that is 4.1 less than what you had even though you added an engine. It's not quite that simple due to ratio of engineers per volume of engine, but you get the general idea behind it.

Now I don't remember if top speed is linked to volume of the engines or the volume and thrust to weight ratio.

(Keep in mind this is somewhat counter intuitive to real life, where currently used forms of thrust - Non-theoretical or sci-fi - in space travel are more functions of surface and not volume.)

Edited for clarification.