Optimize a termination test in str_map_function

l3kernel/l3str.dtx

@@ -1315,9 +1315,14 @@
 %   following space by a braced space and a further call to itself.
 %   These are received by \cs{@@_map_function:Nn}, which passes
 %   the space to |#1| and calls \cs{@@_map_function:w} to deal with the
-%   next space.  Of course we need to include a trailing space (the
-%   question mark is needed to avoid losing the space when \TeX{}
-%   tokenizes the line).
+%   next space.  The space before the braced space allows to optimize
+%   the \cs{q_recursion_tail} test.  Of course we need to include a
+%   trailing space (the question mark is needed to avoid losing the
+%   space when \TeX{} tokenizes the line).
+%   At the cost of about three more auxiliaries this code could get a $9$
+%   times speed up by testing only every $9$-th character for whether it
+%   is \cs{q_recursion_tail} (also by converting $9$ spaces at a time in
+%   the \cs{str_map_function:nN} case).
 %    \begin{macrocode}
 \cs_new:Npn \str_map_function:nN #1#2
   {
@@ -1329,10 +1334,13 @@
   }
 \cs_new:Npn \str_map_function:NN
   { \exp_args:No \str_map_function:nN }
-\cs_new:Npn \@@_map_function:w #1 ~ { #1 { { ~ } \@@_map_function:w } }
+\cs_new:Npn \@@_map_function:w #1 ~
+  { #1 { ~ { ~ } \@@_map_function:w } }
 \cs_new:Npn \@@_map_function:Nn #1#2
   {
-    \__quark_if_recursion_tail_break:nN {#2} \str_map_break:
+    \if_meaning:w \q_recursion_tail #2
+      \exp_after:wN \str_map_break:
+    \fi:
     #1 #2 \@@_map_function:Nn #1
   }
 \cs_generate_variant:Nn \str_map_function:NN { c }