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

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.