* Turn off synchronisation between C and C++ I/O functions. This

gives a huge speedup in operations that read or write from standard
  input/output.  (So libstdc++'s I/O isn't that bad, you just have to
  call std::ios::sync_with_stdio(false).)  For instance, `nix-store
  --register-substitutes' went from 1.4 seconds to 0.1 seconds on a
  certain input.  Another victory for Valgrind.

src/libmain/shared.cc

@@ -225,6 +225,8 @@ int main(int argc, char * * argv)
     std::cerr.rdbuf()->pubsetbuf(buf, sizeof(buf));
 #endif
 
+    std::ios::sync_with_stdio(false);
+
     try {
         try {
             initAndRun(argc, argv);