Well, as of the time being, I believe they have set a speed limit on ships, no matter the size nor amount of engines, it is set to around 49.9-50.
Imo, this is to make sure that the current ALPHA elements of ship animation, movement and design does not bug out or cause massive lag while traveling.(Clipping in other videos games of high-velocity entities)I believe it has to do with the physical limitations of the current engine and setup of assets.
Hope this explained what you were wondering about.