astropy: Significant performance hit upgrading Sphinx 3.2.1 to 3.3.0

[pllim]

Describe the bug

Build time increased from 550-ish seconds to 1400-ish seconds when Sphinx was upgraded from 3.2.1 to 3.3.0.

To Reproduce

I am not sure how to reproduce this as the astropy package has a lot of documentation and uses its own sphinx-astropy and sphinx-automodapi on top of vanilla Sphinx.

Expected behavior

Upgrading Sphinx minor version should not have such significant slowdown.

Your project

https://github.com/astropy/astropy/ and https://readthedocs.org/projects/astropy/

Screenshots

N/A

Environment info

• OS: Linux
• Python version: 3.8
• Sphinx version: 3.2.1, 3.3.0
• Sphinx extensions: Various, see above
• Extra tools: RTD

Additional context

• MNT: Pin sphinx to avoid very slow doc build on RTD astropy/astropy#11014 (particularly MNT: Pin sphinx to avoid very slow doc build on RTD astropy/astropy#11014 (comment))
• Build timed out for no apparent reason readthedocs/readthedocs.org#7648

Any insight would be greatly appreciated. Thank you!

[tk0miya]

I fixed some slowdown bugs in 3.3.1. Could you try the new one, please?

[pllim]

Yes, seems fixed. Thanks!