I attached a minimal C extension module that can be used to demonstrate the performance degradation from Python 3.7.0a3 to 3.7.0a4.

Build the extension with `py setup.py build_ext --inplace`, then run the following code on Python 3.7.0a3 to 3.7.0a4:

```
import time
from test_isdigit import test_isdigit

start_time = time.time()
test_isdigit()
print(time.time() - start_time)
```

On my Windows 10 Pro WS system, the timings are:

Python 3.7.0a3: ~0.0156
Python 3.7.0a4: ~0.3281


I would expect that other locale aware functions in the UCRT are also affected but I have not tested that.