input: Write a python function to optimize a slow running function by using the built-in `sum` function, vectorization with NumPy, or Cython. 
  assert optimized_func([1, 2, 3, 4, 5]) == 15
  assert optimized_func([10, 20, 30, 40, 50]) == 150
  assert optimized_func([-1, -2, -3, -4, -5]) == -15
output: def optimized_func(arr):
      return sum(arr)