fix formatting and add comment

src/SFML/System/Vector2.cpp

@@ -107,6 +107,7 @@ T Vector2<T>::length() const
 {
     static_assert(std::is_floating_point_v<T>, "Vector2::length() is only supported for floating point types");
 
+    // don't use std::hypot because of slow performance
     return std::sqrt(x * x + y * y);
 }
 

src/SFML/System/Vector3.cpp

@@ -48,6 +48,7 @@ T Vector3<T>::length() const
 {
     static_assert(std::is_floating_point_v<T>, "Vector3::length() is only supported for floating point types");
 
+    // don't use std::hypot because of slow performance
     return std::sqrt(x * x + y * y + z * z);
 }