Numpy

import math
import time
import random
import numpy as np

random.seed(time.time())
t1 = time.time()

v = np.vectorize(lambda b: math.pow(b, 1.546) if b > 0.5 else math.pow(b, 0.434232))
a = np.arctan(np.cos(v(np.random.random((4999999))) + np.arange(4999999)))

t2 = time.time()
print t2 - t1