diff --git a/setup.cfg b/setup.cfg
index 7f570c745..224ebf11b 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -103,6 +103,8 @@ filterwarnings =
     ignore:'resetCache' deprecated:DeprecationWarning:matplotlib
     ignore:'enablePackrat' deprecated:DeprecationWarning:matplotlib
     ignore:'asyncio.AbstractEventLoopPolicy' is deprecated:DeprecationWarning:pytest_asyncio
+markers =
+    slow: long-running test cell (typical: golden-corpus fixtures behind a heavy codec or large pixel count). PR CI can skip with `-m "not slow"`; nightly / release runs use no filter. See xrspatial/geotiff/tests/golden_corpus/_marks.py for the corpus-side helper.
 
 [isort]
 line_length = 100
diff --git a/xrspatial/geotiff/tests/golden_corpus/README.md b/xrspatial/geotiff/tests/golden_corpus/README.md
index 411bc31a8..f118d9285 100644
--- a/xrspatial/geotiff/tests/golden_corpus/README.md
+++ b/xrspatial/geotiff/tests/golden_corpus/README.md
@@ -46,6 +46,27 @@ iteration, and mtimes normalised to a fixed epoch.
 The schema is documented in the comments at the top of `manifest.yaml`
 and enforced by `generate.validate()`.
 
+## Fast / slow split
+
+Each fixture's `tags:` list controls whether it runs in the PR CI fast
+lane. A fixture is **fast** if `"fast"` appears in its `tags`. Everything
+else picks up `pytest.mark.slow` automatically via the helper in
+`_marks.py`, which the per-backend test modules consume from their
+`_build_param`.
+
+* `pytest`: runs every cell, fast and slow.
+* `pytest -m "not slow"`: PR fast lane; skips heavy cells.
+* `pytest -m slow`: only the slow cells, e.g. for a nightly job that
+  exercises the long tail.
+
+Today most shipped fixtures carry `fast`. The six `compression_*`
+fixtures in the manifest do not, so `pytest -m "not slow"` deselects
+them. A one-line manifest edit per fixture would move them into the
+fast lane if the team decides that is the right calibration. Future
+heavier fixtures (large COGs, jpeg2000, multi-source VRTs) drop in
+behind the same boundary without re-plumbing each backend test
+module.
+
 ## What is deliberately not in this PR
 
 * Real fixture files. Phase 2 PRs add them in batches (tiled/stripped,
diff --git a/xrspatial/geotiff/tests/golden_corpus/_marks.py b/xrspatial/geotiff/tests/golden_corpus/_marks.py
new file mode 100644
index 000000000..7ec3cc64f
--- /dev/null
+++ b/xrspatial/geotiff/tests/golden_corpus/_marks.py
@@ -0,0 +1,62 @@
+"""Fast / slow pytest marker helper for the golden-corpus matrix
+(issue #1930, phase 4 PR 1).
+
+Each manifest fixture carries a ``tags`` list. Fixtures tagged ``fast``
+run in the PR CI fast lane; everything else is treated as slow and
+gets ``pytest.mark.slow`` attached so PR CI can opt out via
+``pytest -m "not slow"``. Nightly / release CI runs without the filter
+and exercises everything.
+
+Today most shipped fixtures carry ``fast``. The six ``compression_*``
+fixtures in the manifest do not, so they land in the slow lane and
+``pytest -m "not slow"`` deselects them. A one-line manifest edit per
+fixture would move them to the fast lane if the team decides that is
+the right calibration. Future heavier fixtures (large COGs,
+multi-source VRTs, jpeg2000 cells) will drop in behind the same
+boundary without each backend test module re-implementing it.
+
+Usage from a backend test module::
+
+    from xrspatial.geotiff.tests.golden_corpus._marks import (
+        fast_slow_marks_for,
+    )
+
+    def _build_param(entry):
+        marks = list(fast_slow_marks_for(entry))
+        if entry["id"] in _PARITY_GAPS:
+            marks.append(pytest.mark.xfail(...))
+        return pytest.param(entry, id=entry["id"], marks=marks)
+
+``fast_slow_marks_for`` is a generator, so chaining with other marks
+is just a ``list(...) + [extra_mark]`` away.
+"""
+from __future__ import annotations
+
+from typing import Any
+
+import pytest
+
+
+_FAST_TAG = "fast"
+
+
+def is_fast(entry: dict[str, Any]) -> bool:
+    """Return True when the manifest entry is in the fast lane.
+
+    The contract: a fixture is fast iff its ``tags`` list contains the
+    literal string ``"fast"``. Missing or empty ``tags`` count as slow,
+    on the theory that an untagged fixture is one a contributor forgot
+    to triage rather than one that is intentionally cheap.
+    """
+    tags = entry.get("tags") or []
+    return _FAST_TAG in tags
+
+
+def fast_slow_marks_for(entry: dict[str, Any]) -> list[pytest.MarkDecorator]:
+    """Return the slow mark (in a list) when the entry is not fast.
+
+    Returns ``[pytest.mark.slow]`` for slow fixtures and ``[]`` for fast
+    ones, so the caller can splat the result into its ``marks=`` list
+    without an empty-mark guard or a generator-to-list conversion.
+    """
+    return [pytest.mark.slow] if not is_fast(entry) else []
diff --git a/xrspatial/geotiff/tests/test_golden_corpus_eager_numpy_1930.py b/xrspatial/geotiff/tests/test_golden_corpus_eager_numpy_1930.py
index a48c3c61b..8b4cea535 100644
--- a/xrspatial/geotiff/tests/test_golden_corpus_eager_numpy_1930.py
+++ b/xrspatial/geotiff/tests/test_golden_corpus_eager_numpy_1930.py
@@ -61,6 +61,9 @@
 
 from xrspatial.geotiff import open_geotiff  # noqa: E402
 from xrspatial.geotiff.tests.golden_corpus import generate  # noqa: E402
+from xrspatial.geotiff.tests.golden_corpus._marks import (  # noqa: E402
+    fast_slow_marks_for,
+)
 from xrspatial.geotiff.tests.golden_corpus._oracle import (  # noqa: E402
     compare_to_oracle,
 )
@@ -126,26 +129,22 @@ def _is_lossy(entry: dict) -> bool:
 
 
 def _build_param(entry: dict) -> pytest.param:
-    """Wrap a fixture entry in a ``pytest.param`` with the right mark.
+    """Wrap a fixture entry in a ``pytest.param`` with the right marks.
 
     Real parity gaps get ``xfail(strict=True)`` so the test surfaces a hard
     failure the day the gap closes. The MinIsWhite cell gets a plain skip
-    because the divergence is intentional.
+    because the divergence is intentional. Non-fast fixtures additionally
+    pick up ``pytest.mark.slow`` from the corpus helper.
     """
     fid = entry["id"]
+    marks = list(fast_slow_marks_for(entry))
     if fid in _PARITY_GAPS:
-        return pytest.param(
-            entry,
-            id=fid,
-            marks=pytest.mark.xfail(reason=_PARITY_GAPS[fid], strict=True),
-        )
-    if fid in _INTENTIONAL_SKIPS:
-        return pytest.param(
-            entry,
-            id=fid,
-            marks=pytest.mark.skip(reason=_INTENTIONAL_SKIPS[fid]),
+        marks.append(
+            pytest.mark.xfail(reason=_PARITY_GAPS[fid], strict=True)
         )
-    return pytest.param(entry, id=fid)
+    elif fid in _INTENTIONAL_SKIPS:
+        marks.append(pytest.mark.skip(reason=_INTENTIONAL_SKIPS[fid]))
+    return pytest.param(entry, id=fid, marks=marks)
 
 
 _FIXTURES = _resolved_fixtures()