Optimized

import math
import time
import random

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

a = [math.atan(math.cos((math.pow(b, 1.546) if b > 0.5 else math.pow(b, 0.434232)) + i)) for i,b in enumerate([random.random() for j in xrange(4999999)])]

t2 = time.time()
print t2 - t1