diff --git a/online-tests/conftest.py b/online-tests/conftest.py
index 64b0f7ed..5f4eb512 100644
--- a/online-tests/conftest.py
+++ b/online-tests/conftest.py
@@ -18,6 +18,9 @@ def pytest_addoption(parser):
 
 def pytest_configure(config):
     config._benchresults = {}
+    config.addinivalue_line(
+        "markers", "slow: mark test to require --slow option to run"
+    )
 
 
 def pytest_runtest_setup(item):
diff --git a/online-tests/pytest.ini b/online-tests/pytest.ini
index 2ac3bab1..b096b773 100644
--- a/online-tests/pytest.ini
+++ b/online-tests/pytest.ini
@@ -1,3 +1,2 @@
 [pytest]
 addopts = -vrsx --strict-markers
-markers = slow: mark test as slow (requires --slow option to run)