In hover vehicles, your top speed is limited separately for forwards & backwards and left & right. However, the limit in either is the same; the max sideways speed is equal to the maximum forward speed.

As such, it is faster to fly diagonally in a hover vehicle than it is to fly forwards, with diagonal speed working out to approximately 1.4x forward speed (root 2 times, to be specific).

You can use the Jump Thruster (E) to go even faster. To do this, fly diagonally, angle the cursor to the opposite corner to the direction you are flying (if using forwards & left, point the cursor to the bottom right), and hold E. From what I can tell, the jump thruster doesn't have the same top speed as the forward or sideways limit, giving a top speed somewhere between 1.5x and 1.7x normal speed.