What happened?

Commit a13a255#diff-c73f63e529ab4065cca1aa23d93c91a3b060f314e2e498b708b90fef87141a88 which attempts to fix some type hint issues also introduced a major performance regression in Coordinate.to_index() (mostly felt in to_dataframe()) by changing the code_list variable being cache-friendly ndarray to Python lists.

What did you expect to happen?

I expect this to be near instantaneous like it was in 2025.8.0 but now it takes close to 10s on my machine.

Minimal Complete Verifiable Example

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "xarray[complete]@git+https://github.com/pydata/xarray.git@main",
# ]
# ///
#
# This script automatically imports the development branch of xarray to check for issues.
# Please delete this header if you have _not_ tested this script with `uv run`!

import xarray as xr
xr.show_versions()

import numpy as np
import xarray as xr

da = xr.DataArray(np.arange(100 * 2000 * 300).reshape(100, 2000, 300), name="foo")
da.to_dataframe()

Steps to reproduce

No response

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment