PERF/QST: Why isn't fillna cythonized?
Looking at the fillna code, it looks like a mask is generated and then the fill value is then put in using putmask. Wouldn't it be faster if we skipped the mask generation and just put in nan values directly by looping over in Cython.
https://github.com/pandas-dev/pandas/blob/98e22297bb66f81ff4c1d4f1c3277cfe8c6b9ce7/pandas/core/internals/blocks.py#L430-L470
I can take this issue up if this is the correct approach