--- a/dom/bindings/parser/WebIDL.py
+++ b/dom/bindings/parser/WebIDL.py
@@ -5000,18 +5000,24 @@ class Parser(Tokenizer):
         else:
             raise WebIDLError("invalid syntax", [Location(self.lexer, p.lineno, p.lexpos, self._filename)])
 
     def __init__(self, outputdir='', lexer=None):
         Tokenizer.__init__(self, outputdir, lexer)
         self.parser = yacc.yacc(module=self,
                                 outputdir=outputdir,
                                 tabmodule='webidlyacc',
-                                errorlog=yacc.NullLogger(),
-                                picklefile='WebIDLGrammar.pkl')
+                                errorlog=yacc.NullLogger()
+                                # Pickling the grammar is a speedup in
+                                # some cases (older Python?) but a
+                                # significant slowdown in others.
+                                # We're not pickling for now, until it
+                                # becomes a speedup again.
+                                # , picklefile='WebIDLGrammar.pkl'
+                            )
         self._globalScope = IDLScope(BuiltinLocation("<Global Scope>"), None, None)
         self._installBuiltins(self._globalScope)
         self._productions = []
 
         self._filename = "<builtin>"
         self.lexer.input(Parser._builtins)
         self._filename = None