diff --git a/etc/NEWS b/etc/NEWS
index 1519ee8e257..77b1043a08d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -305,6 +305,11 @@ on the header lines are now these two: the selected window uses
 
 ** In 'customize-face', the "Font family" attribute now supports completion.
 
+** 'process-adaptive-read-buffering' is now nil by default.
+Setting this variable to a non-nil value reduces performance and leads
+to wrong results in some cases.  We believe that it is no longer useful;
+please contact us if you still need it for some reason.
+
 
 * Editing Changes in Emacs 31.1
 
diff --git a/src/process.c b/src/process.c
index 807f06f990b..e61ec425f7e 100644
--- a/src/process.c
+++ b/src/process.c
@@ -8878,7 +8878,7 @@ allow them to produce more output before Emacs tries to read it.
 If the value is t, the delay is reset after each write to the process; any other
 non-nil value means that the delay is not reset on write.
 The variable takes effect when `start-process' is called.  */);
-  Vprocess_adaptive_read_buffering = Qt;
+  Vprocess_adaptive_read_buffering = Qnil;
 
   DEFVAR_BOOL ("process-prioritize-lower-fds", process_prioritize_lower_fds,
 	       doc: /* Whether to start checking for subprocess output from first file descriptor.