Microbenchmark results:

$ ./python -m perf timeit --compare-to=../cpython-release/python -s 'from itertools import repeat' 'list(repeat(None, 1000000))'
/home/serhiy/py/cpython-release/python: ..................... 3.79 ms +- 0.09 ms
/home/serhiy/py/cpython-iter/python: ..................... 4.14 ms +- 0.07 ms

Mean +- std dev: [/home/serhiy/py/cpython-release/python] 3.79 ms +- 0.09 ms -> [/home/serhiy/py/cpython-iter/python] 4.14 ms +- 0.07 ms: 1.09x slower (+9%)


$ ./python -m perf timeit --compare-to=../cpython-release/python -s 'from itertools import cycle, islice' 'list(islice(cycle(range(1000)), 1000000))'
/home/serhiy/py/cpython-release/python: ..................... 6.88 ms +- 0.30 ms
/home/serhiy/py/cpython-iter/python: ..................... 6.87 ms +- 0.26 ms

Mean +- std dev: [/home/serhiy/py/cpython-release/python] 6.88 ms +- 0.30 ms -> [/home/serhiy/py/cpython-iter/python] 6.87 ms +- 0.26 ms: 1.00x faster (-0%)
Not significant!


$ ./python -m perf timeit --compare-to=../cpython-release/python -s 'from itertools import count, islice' 'list(islice(count(), 1000000))'
/home/serhiy/py/cpython-release/python: ..................... 26.1 ms +- 0.6 ms
/home/serhiy/py/cpython-iter/python: ..................... 26.3 ms +- 0.6 ms

Mean +- std dev: [/home/serhiy/py/cpython-release/python] 26.1 ms +- 0.6 ms -> [/home/serhiy/py/cpython-iter/python] 26.3 ms +- 0.6 ms: 1.01x slower (+1%)
Not significant!


$ ./python -m perf timeit --compare-to=../cpython-release/python -s 'from itertools import product' 'list(product(range(100), repeat=3))'
/home/serhiy/py/cpython-release/python: ..................... 80.2 ms +- 3.2 ms
/home/serhiy/py/cpython-iter/python: ..................... 80.2 ms +- 1.7 ms

Mean +- std dev: [/home/serhiy/py/cpython-release/python] 80.2 ms +- 3.2 ms -> [/home/serhiy/py/cpython-iter/python] 80.2 ms +- 1.7 ms: 1.00x faster (-0%)
Not significant!


$ ./python -m perf timeit --compare-to=../cpython-release/python -s 'from itertools import combinations' 'list(combinations(range(23), 10))'
/home/serhiy/py/cpython-release/python: ..................... 177 ms +- 14 ms
/home/serhiy/py/cpython-iter/python: ..................... 169 ms +- 4 ms

Mean +- std dev: [/home/serhiy/py/cpython-release/python] 177 ms +- 14 ms -> [/home/serhiy/py/cpython-iter/python] 169 ms +- 4 ms: 1.05x faster (-4%)


The only significant slowdown is for repeat(). But there is possibility to optimize this one by reusing an existing counter.