diff --git a/doc/arm/reference.rst b/doc/arm/reference.rst
index 712d284ed9..38396f03a5 100644
--- a/doc/arm/reference.rst
+++ b/doc/arm/reference.rst
@@ -7552,6 +7552,8 @@ the zone's filename, unless :any:`inline-signing` is enabled.
 
        The daemon replies with a four-byte value in network byte order, containing either 0 or 1; 0 indicates that the specified update is not permitted, and 1 indicates that it is.
 
+       .. warning:: The external daemon must not delay communication. This policy is evaluated synchronously; any wait period negatively affects :iscman:`named` performance.
+
 .. _multiple_views:
 
 Multiple Views