Performance regression in fillna
https://pandas.pydata.org/speed/pandas/index.html#replace.FillNa.time_fillna?python=3.8&Cython=0.29.21&p-inplace=False&commits=2eb353063d6bc4f2ed22e9943d26465e284b8393-68f53cd9e556698fe786aefe90466f233eb55841

https://github.com/pandas-dev/pandas/compare/2eb353063d6bc4f2ed22e9943d26465e284b8393...68f53cd9e556698fe786aefe90466f233eb55841

- [ ] I have checked that this issue has not already been reported.

- [ ] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

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