* 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);

src/nix-store/main.cc

@@ -632,7 +632,7 @@ static void opDump(Strings opFlags, Strings opArgs)
 }
 
 
-/* A source that read restore intput to stdin. */
+/* A source that reads restore input from stdin. */
 struct StdinSource : RestoreSource
 {
     virtual void operator () (unsigned char * data, unsigned int len)