diff --git a/scico/linop/xray/_xray.py b/scico/linop/xray/_xray.py
index 04bdabe8..75f035af 100644
--- a/scico/linop/xray/_xray.py
+++ b/scico/linop/xray/_xray.py
@@ -235,6 +235,9 @@ def _calc_weights(
 class XRayTransform3D(LinearOperator):
     r"""General-purpose, 3D, parallel ray X-ray projector.
 
+    Warning: This class is experimental and may be up to ten times slower
+    than :class:`scico.linop.xray.astra.XRayTransform3D`.
+
     For each view, the projection geometry is specified by an array
     with shape (2, 4) that specifies a :math:`2 \times 3` projection
     matrix and a :math:`2 \times 1` offset vector. Denoting the matrix