diff --git a/Lib/_pyio.py b/Lib/_pyio.py
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -944,6 +944,12 @@ class BufferedReader(_BufferedIOMixin):
         # Special case for when the number of bytes to read is unspecified.
         if n is None or n == -1:
             self._reset_read_buf()
+            if hasattr(self.raw, 'readall'):
+                chunk = self.raw.readall()
+                if chunk is None:
+                    return buf[pos:] or None
+                else:
+                    return buf[pos:] + chunk
             chunks = [buf[pos:]]  # Strip the consumed bytes.
             current_size = 0
             while True:
diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
--- a/Modules/_io/_iomodule.c
+++ b/Modules/_io/_iomodule.c
@@ -36,6 +36,7 @@ PyObject *_PyIO_str_nl;
 PyObject *_PyIO_str_read;
 PyObject *_PyIO_str_read1;
 PyObject *_PyIO_str_readable;
+PyObject *_PyIO_str_readall;
 PyObject *_PyIO_str_readinto;
 PyObject *_PyIO_str_readline;
 PyObject *_PyIO_str_reset;
@@ -767,6 +768,8 @@ PyInit__io(void)
         goto fail;
     if (!(_PyIO_str_readable = PyUnicode_InternFromString("readable")))
         goto fail;
+    if (!(_PyIO_str_readall = PyUnicode_InternFromString("readall")))
+        goto fail;
     if (!(_PyIO_str_readinto = PyUnicode_InternFromString("readinto")))
         goto fail;
     if (!(_PyIO_str_readline = PyUnicode_InternFromString("readline")))
diff --git a/Modules/_io/_iomodule.h b/Modules/_io/_iomodule.h
--- a/Modules/_io/_iomodule.h
+++ b/Modules/_io/_iomodule.h
@@ -155,6 +155,7 @@ extern PyObject *_PyIO_str_nl;
 extern PyObject *_PyIO_str_read;
 extern PyObject *_PyIO_str_read1;
 extern PyObject *_PyIO_str_readable;
+extern PyObject *_PyIO_str_readall;
 extern PyObject *_PyIO_str_readinto;
 extern PyObject *_PyIO_str_readline;
 extern PyObject *_PyIO_str_reset;
diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -1407,32 +1407,57 @@ static PyObject *
 _bufferedreader_read_all(buffered *self)
 {
     Py_ssize_t current_size;
-    PyObject *res, *data = NULL;
-    PyObject *chunks = PyList_New(0);
-
-    if (chunks == NULL)
-        return NULL;
+    PyObject *res, *data = NULL, *chunk, *chunks;
 
     /* First copy what we have in the current buffer. */
     current_size = Py_SAFE_DOWNCAST(READAHEAD(self), Py_off_t, Py_ssize_t);
     if (current_size) {
         data = PyBytes_FromStringAndSize(
             self->buffer + self->pos, current_size);
-        if (data == NULL) {
-            Py_DECREF(chunks);
+        if (data == NULL)
             return NULL;
-        }
     }
     _bufferedreader_reset_buf(self);
     /* We're going past the buffer's bounds, flush it */
     if (self->writable) {
         res = _bufferedwriter_flush_unlocked(self, 1);
-        if (res == NULL) {
-            Py_DECREF(chunks);
+        if (res == NULL)
+            return NULL;
+        Py_CLEAR(res);
+    }
+
+    if (PyObject_HasAttr(self->raw, _PyIO_str_readall)) {
+        chunk = PyObject_CallMethodObjArgs(self->raw, _PyIO_str_readall, NULL);
+        if (chunk == NULL)
+            return NULL;
+        if (chunk != Py_None && !PyBytes_Check(chunk)) {
+            Py_XDECREF(data);
+            Py_DECREF(chunk);
+            PyErr_SetString(PyExc_TypeError, "readall() should return bytes");
             return NULL;
         }
-        Py_CLEAR(res);
+        if (chunk == Py_None) {
+            if (current_size == 0)
+                return chunk;
+            else {
+                Py_DECREF(chunk);
+                return data;
+            }
+        }
+        else if (current_size) {
+            PyBytes_Concat(&data, chunk);
+            Py_DECREF(chunk);
+            if (data == NULL)
+                return NULL;
+            return data;
+        } else
+            return chunk;
     }
+
+    chunks = PyList_New(0);
+    if (chunks == NULL)
+        return NULL;
+
     while (1) {
         if (data) {
             if (PyList_Append(chunks, data) < 0) {