We know for a fact in "Q Who" and "The Best of Both Worlds" that a Borg ship is more than capable of outrunning the Enterprise at top speeds.
So how come in the film "First Contact" the Borg cube isn't going neeoww whiz pow bang whoosh! ? Because they knew it would look stupid. This is how Hollywood used to work before they let pajeets do everything. This shot is almost reminiscent of King Kong. A huge gorilla swiping at tiny fighter planes whizzing around him.
>but it's not REALISTIC!!!
That's not the point. The point is to let the audience know that Borg ships are massive, while the Federation ships are much, much smaller in comparison.