If the game is running acceptably in one interpreter (Gargoyle, which will use Git or Glulxe under the hood) and slow in another (the one built into Inform 7), my first guess would be that the accelerated opcode support is only active in the former and not the latter. Is this on Windows, Mac or Linux? On Windows I know for sure that the accelerated opcodes are active in the interpreter in the I7 front end, but I’ve no idea on Mac. One way to check this would be a small Inform 6 inclusion to print out the result of calling the gestalt function with the appropriate selector and running the result in different places.
The IDE interpreter on OS X is markedly slower for many operations than external interpreters. This may have something to do with how features like the skein is implemented, among other factors such as those mentioned by others in this thread. In any case, you’re definitely not alone in observing this.