gh-150860: Skip the whitespace scan in json.loads() when there is none

decode() ran a whitespace-skipping regex at both ends of every document
even though most have none. Skip the leading match when the document does
not start with whitespace and the trailing match when the parse already
consumed the whole string. Documents with surrounding whitespace keep the
original behavior and output is unchanged.

Author: gaborbernat

Lib/json/decoder.py

@@ -355,10 +355,16 @@ def decode(self, s, _w=WHITESPACE.match):
         containing a JSON document).
 
         """
-        obj, end = self.raw_decode(s, idx=_w(s, 0).end())
-        end = _w(s, end).end()
+        # Skip the WHITESPACE.match() call (and its match-object allocation)
+        # for the common case where there is no leading whitespace.
+        idx = _w(s, 0).end() if s and s[0] in ' \t\n\r' else 0
+        obj, end = self.raw_decode(s, idx=idx)
+        # Likewise avoid the trailing-whitespace match when the parse already
+        # consumed the whole string.
         if end != len(s):
-            raise JSONDecodeError("Extra data", s, end)
+            end = _w(s, end).end()
+            if end != len(s):
+                raise JSONDecodeError("Extra data", s, end)
         return obj
 
     def raw_decode(self, s, idx=0):

Misc/NEWS.d/next/Library/2026-06-03-07-31-11.gh-issue-150860.8VbmHU.rst

@@ -0,0 +1,3 @@
+Speed up :func:`json.loads` for documents without leading or trailing
+whitespace by skipping the whitespace scan in that common case. Patch by Bernát
+Gábor.