PERF: CategoricalIndex.get_indexer
- [x] closes #42249
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

```
import pandas as pd
import string, itertools
data_unique = pd.CategoricalIndex(
            ["".join(perm) for perm in itertools.permutations(string.printable, 3)]
)
cat_list = ["a", "c"]

%timeit data_unique.get_indexer(cat_list)
106 µs ± 6.14 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)  # <- PR
39 ms ± 616 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)  # <- 1.2.4
290 ms ± 3.9 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)  # <- master
```
Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.3.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 73ed010db5292c31b3bb562010acacaf4247ce70
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #42270: PERF: CategoricalIndex.get_indexer'
```

4. Push to a named branch :

```
git push YOURFORK 1.3.x:auto-backport-of-pr-42270-on-1.3.x
```

5. Create a PR against branch 1.3.x, I would have named this PR:

> "Backport PR #42270 on branch 1.3.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 @meeseeksdev backport 1.3.x cc @jbrockmendel guess this won't backport easily Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.3.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 73ed010db5292c31b3bb562010acacaf4247ce70
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #42270: PERF: CategoricalIndex.get_indexer'
```

4. Push to a named branch :

```
git push YOURFORK 1.3.x:auto-backport-of-pr-42270-on-1.3.x
```

5. Create a PR against branch 1.3.x, I would have named this PR:

> "Backport PR #42270 on branch 1.3.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                