import time


n = 40000000


def first():
    d = dict(enumerate(range(n)))

    t = time.clock()
    i = iter(d)
    print(i)
    print('Without: %i' % ((time.clock() - t)*1e6))



def second():
    d = dict(enumerate(range(n)))

    t = time.clock()
    i = iter(d.keys())
    print(i)
    print('With.keys():  %i' % ((time.clock() - t)*1e6))



first()
second()