Numpy Optimized

import math
import time
import random
import numpy as np

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

l = np.random.random((4999999))
a = np.arctan(np.cos(np.concatenate((np.power(l[l > 0.5], 1.546), np.power(l[l <= 0.5], 0.434232))) + np.arange(4999999)))

t2 = time.time()
print t2 - t1