diff --git a/pandas/tests/groupby/aggregate/test_numba.py b/pandas/tests/groupby/aggregate/test_numba.py
index d585cc1648c5a..10ed32a334d18 100644
--- a/pandas/tests/groupby/aggregate/test_numba.py
+++ b/pandas/tests/groupby/aggregate/test_numba.py
@@ -13,6 +13,8 @@
 )
 import pandas._testing as tm
 
+pytestmark = pytest.mark.single_cpu
+
 
 @td.skip_if_no("numba")
 def test_correct_function_signature():
diff --git a/pandas/tests/groupby/test_numba.py b/pandas/tests/groupby/test_numba.py
index 4eb7b6a7b5bea..867bdbf583388 100644
--- a/pandas/tests/groupby/test_numba.py
+++ b/pandas/tests/groupby/test_numba.py
@@ -8,6 +8,8 @@
 )
 import pandas._testing as tm
 
+pytestmark = pytest.mark.single_cpu
+
 
 @td.skip_if_no("numba")
 @pytest.mark.filterwarnings("ignore")
diff --git a/pandas/tests/groupby/transform/test_numba.py b/pandas/tests/groupby/transform/test_numba.py
index 00ff391199652..557e132567a2e 100644
--- a/pandas/tests/groupby/transform/test_numba.py
+++ b/pandas/tests/groupby/transform/test_numba.py
@@ -11,6 +11,8 @@
 )
 import pandas._testing as tm
 
+pytestmark = pytest.mark.single_cpu
+
 
 @td.skip_if_no("numba")
 def test_correct_function_signature():
diff --git a/pandas/tests/io/parser/test_multi_thread.py b/pandas/tests/io/parser/test_multi_thread.py
index ab278470934a5..562b99090dfab 100644
--- a/pandas/tests/io/parser/test_multi_thread.py
+++ b/pandas/tests/io/parser/test_multi_thread.py
@@ -15,7 +15,11 @@
 
 # We'll probably always skip these for pyarrow
 # Maybe we'll add our own tests for pyarrow too
-pytestmark = pytest.mark.usefixtures("pyarrow_skip")
+pytestmark = [
+    pytest.mark.single_cpu,
+    pytest.mark.slow,
+    pytest.mark.usefixtures("pyarrow_skip"),
+]
 
 
 def _construct_dataframe(num_rows):
@@ -40,7 +44,6 @@ def _construct_dataframe(num_rows):
     return df
 
 
-@pytest.mark.slow
 def test_multi_thread_string_io_read_csv(all_parsers):
     # see gh-11786
     parser = all_parsers
@@ -135,7 +138,6 @@ def reader(arg):
     return final_dataframe
 
 
-@pytest.mark.slow
 def test_multi_thread_path_multipart_read_csv(all_parsers):
     # see gh-11786
     num_tasks = 4
diff --git a/pandas/tests/io/pytables/test_categorical.py b/pandas/tests/io/pytables/test_categorical.py
index bb95762950d8e..b227c935c2b62 100644
--- a/pandas/tests/io/pytables/test_categorical.py
+++ b/pandas/tests/io/pytables/test_categorical.py
@@ -14,9 +14,7 @@
     ensure_clean_store,
 )
 
-pytestmark = [
-    pytest.mark.single_cpu,
-]
+pytestmark = pytest.mark.single_cpu
 
 
 def test_categorical(setup_path):
diff --git a/pandas/tests/io/test_html.py b/pandas/tests/io/test_html.py
index e54a23b1f8ef6..5c6c33de5ac5f 100644
--- a/pandas/tests/io/test_html.py
+++ b/pandas/tests/io/test_html.py
@@ -1347,6 +1347,7 @@ def __iter__(self) -> Iterator:
         assert self.read_html(bad)
 
     @pytest.mark.slow
+    @pytest.mark.single_cpu
     def test_importcheck_thread_safety(self, datapath):
         # see gh-16928
 
diff --git a/pandas/tests/io/test_user_agent.py b/pandas/tests/io/test_user_agent.py
index 06051a81679fa..b9d3a20b2ecea 100644
--- a/pandas/tests/io/test_user_agent.py
+++ b/pandas/tests/io/test_user_agent.py
@@ -17,10 +17,13 @@
 import pandas as pd
 import pandas._testing as tm
 
-pytestmark = pytest.mark.skipif(
-    is_ci_environment(),
-    reason="GH 45651: This test can hang in our CI min_versions build",
-)
+pytestmark = [
+    pytest.mark.single_cpu,
+    pytest.mark.skipif(
+        is_ci_environment(),
+        reason="GH 45651: This test can hang in our CI min_versions build",
+    ),
+]
 
 
 class BaseUserAgentResponder(http.server.BaseHTTPRequestHandler):
diff --git a/pandas/tests/window/test_numba.py b/pandas/tests/window/test_numba.py
index dbafcf12513e0..f5ef6a00e0b32 100644
--- a/pandas/tests/window/test_numba.py
+++ b/pandas/tests/window/test_numba.py
@@ -17,13 +17,15 @@
 )
 import pandas._testing as tm
 
-# TODO(GH#44584): Mark these as pytest.mark.single_cpu
-pytestmark = pytest.mark.skipif(
-    is_ci_environment() and (is_platform_windows() or is_platform_mac()),
-    reason="On GHA CI, Windows can fail with "
-    "'Windows fatal exception: stack overflow' "
-    "and macOS can timeout",
-)
+pytestmark = [
+    pytest.mark.single_cpu,
+    pytest.mark.skipif(
+        is_ci_environment() and (is_platform_windows() or is_platform_mac()),
+        reason="On GHA CI, Windows can fail with "
+        "'Windows fatal exception: stack overflow' "
+        "and macOS can timeout",
+    ),
+]
 
 
 @pytest.fixture(params=["single", "table"])
diff --git a/pandas/tests/window/test_online.py b/pandas/tests/window/test_online.py
index 875adf6cef4ac..5974de0ae4009 100644
--- a/pandas/tests/window/test_online.py
+++ b/pandas/tests/window/test_online.py
@@ -14,13 +14,15 @@
 )
 import pandas._testing as tm
 
-# TODO(GH#44584): Mark these as pytest.mark.single_cpu
-pytestmark = pytest.mark.skipif(
-    is_ci_environment() and (is_platform_windows() or is_platform_mac()),
-    reason="On GHA CI, Windows can fail with "
-    "'Windows fatal exception: stack overflow' "
-    "and macOS can timeout",
-)
+pytestmark = [
+    pytest.mark.single_cpu,
+    pytest.mark.skipif(
+        is_ci_environment() and (is_platform_windows() or is_platform_mac()),
+        reason="On GHA CI, Windows can fail with "
+        "'Windows fatal exception: stack overflow' "
+        "and macOS can timeout",
+    ),
+]
 
 
 @td.skip_if_no("numba")