Very slow build time with v3.0.1

[jklaise]

Describe the bug
After upgrading to 3.0.1 (and hotfixing m2r bug #7420), the build of the docs takes a very long time (~1500s compared to ~30s on 2.4.4). Most of the time is spent on reading sources which I generate dynamically with sphinxcontrib.apidoc. This extension just calls sphinx-apidoc, I have checked that the problem persists if I call sphinx-apidoc directly to generate sources first and then make html which results in a very slow build.

To Reproduce
To build our docs you also need pandoc which can be installed via package manager such as conda or via the OS one.

Steps to reproduce the behavior:

$ git clone https://github.com/SeldonIO/alibi.git
$ cd alibi
$ pip install -r requirements/requirements_ci.txt
$ pip uninstall m2r
$ pip install m2r @ git+https://github.com/crossnox/m2r@dev#egg=m2r
$ pip install -e .
$ make build_docs

Expected behavior
Docs build in a reasonable amount of time.

Your project
https://github.com/SeldonIO/alibi

Screenshots
If applicable, add screenshots to help explain your problem.

Environment info

• OS: Arch Linux
• Python version: 3.7.3
• Sphinx version: 3.0.1
• Sphinx extensions:
  [
  'sphinx.ext.autodoc',
  'sphinx.ext.doctest',
  'sphinx.ext.intersphinx',
  'sphinx.ext.todo',
  'sphinx.ext.coverage',
  'sphinx.ext.mathjax',
  'sphinx.ext.ifconfig',
  'sphinx.ext.viewcode',
  'sphinx.ext.napoleon',
  'sphinx_autodoc_typehints',
  'sphinxcontrib.apidoc',
  'nbsphinx',
  'nbsphinx_link',
  'm2r'
  ]

Additional context

• Updated m2r to use this hotfix to build docs in the first place: TypeError: add_source_parser() positional arguments error in Sphinx v3.0.0 #7420
• Full conf.py: https://github.com/SeldonIO/alibi/blob/master/doc/source/conf.py

[fragmuffin]

I'm experiencing a similar slowdown on a closed-source project.

extensions = [
    'sphinx.ext.autodoc',
    'sphinx.ext.doctest',
    'sphinx.ext.intersphinx',
    'sphinx.ext.mathjax',
]

Environment

$ lsb_release -d
Description:	Ubuntu 18.04.3 LTS
$ sphinx-build --version
sphinx-build 3.0.1
$ python --version
Python 3.6.8

Search for root cause

With all api docs removed except for the following:

.. automodule:: ucontrol.battery_ctrl
   :members:
   :undoc-members:
   :show-inheritance:

reading sources still takes a few seconds (much longer than it should)

When removing :members: OR :undoc-members: (or both), the slowdown is no longer there.
(removing :show-inheritance: makes no difference)

So, not a root cause, but at least it's a breadcrumb.

edit: I'm also using autodoc_mock_imports (and so is https://github.com/SeldonIO/alibi (see link in original bug report))

[tk0miya]

Fixed by #7504. It will be released as 3.0.2 soon (see #7505).
Thank you for reporting!

[fragmuffin]

Fix confirmed working with my project.
Thank you for the fix!; greatly appreciated 👍