diff --git a/sphinx/testing/fixtures.py b/sphinx/testing/fixtures.py
index c6d4fbb7c31..a62f34694b7 100644
--- a/sphinx/testing/fixtures.py
+++ b/sphinx/testing/fixtures.py
@@ -145,7 +145,6 @@ def app(
     """
     args, kwargs = app_params
     app_ = make_app(*args, **kwargs)
-    app_.env.config.enable_parallel_post_transform = True
     yield app_
 
     print('# testroot:', kwargs.get('testroot', 'root'))
@@ -190,7 +189,6 @@ def make(*args: Any, **kwargs: Any) -> SphinxTestApp:
         kwargs.setdefault('status', status)
         kwargs.setdefault('warning', warning)
         app_: Any = SphinxTestApp(*args, **kwargs)
-        # app_.env.config.enable_parallel_post_transform = True
         apps.append(app_)
         if test_params['shared_result']:
             app_ = SphinxTestAppWrapperForSkipBuilding(app_)
diff --git a/sphinx/testing/util.py b/sphinx/testing/util.py
index c81f8274532..e15a43b4b33 100644
--- a/sphinx/testing/util.py
+++ b/sphinx/testing/util.py
@@ -91,8 +91,8 @@ def __init__(
         status: IO | None = None,
         warning: IO | None = None,
         tags: list[str] | None = None,
-        docutilsconf: str | None = None,
-        parallel: int = 4,
+        docutils_conf: str | None = None,
+        parallel: int = 0,
     ) -> None:
         assert srcdir is not None
 
diff --git a/sphinx/util/parallel.py b/sphinx/util/parallel.py
index 6b8ce39997d..10f8c8945af 100644
--- a/sphinx/util/parallel.py
+++ b/sphinx/util/parallel.py
@@ -149,7 +149,7 @@ def make_chunks(arguments: Sequence[str], nproc: int, maxbatch: int = 10) -> lis
     chunksize = nargs // nproc
     if chunksize >= maxbatch:
         # try to improve batch size vs. number of batches
-        chunksize = int(sqrt(sqrt(nargs / nproc * maxbatch)))
+        chunksize = int(sqrt(nargs / nproc * maxbatch))
     if chunksize == 0:
         chunksize = 1
     nchunks, rest = divmod(nargs, chunksize)
diff --git a/tox.ini b/tox.ini
index 28632ae6508..fb7e344ff0b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -24,7 +24,7 @@ setenv =
     PYTHONWARNINGS = error
     PYTEST_ADDOPTS = {env:PYTEST_ADDOPTS:} --color yes
 commands=
-    python -X dev -X warn_default_encoding -m pytest -n 4 --durations 25 {posargs}
+    python -X dev -X warn_default_encoding -m pytest --durations 25 {posargs}
 
 [testenv:docs]
 basepython = python3