Achieving a rotated Vector magnitude
You're calling Math.sin() and Math.cos() twice as many times as you need to. 

double cosine = Math.cos(radians)
double sine = Math.sin(radians)
Vector2 rotated = new Vector2(x*cosine - y*sine, x*sine + y*cosine)


If the angle never changes, then just save the values cosine and sine so you don't have to compute them over and over again.