diff --git a/src/app.rs b/src/app.rs
index 67b7295..6344f53 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -1473,6 +1473,10 @@ contents of FILE. This option expects the COMMAND program to either be an
 absolute path or to be available in your PATH. An empty string COMMAND
 deactivates this feature.
 
+WARNING: When this flag is set, ripgrep will unconditionally spawn a process
+for every file that is searched. Therefore, this can incur a large performance
+penalty if you don't otherwise need the flexibility offered by this flag.
+
 A preprocessor is not run when ripgrep is searching stdin.
 
 When searching over sets of files that may require one of several decoders