Slowdown on the HD is usually down to either; Bad rip, dash settings or disc fragmentation.
Ok firstly rerip the disc, an original disc, to your HD using DVD2Xbox, HDDloader causes slowdown.
Next delete caches.
Next check your dash settings, if your using exox make sure TSR and IGR are disabled, if you are using another dash just turn everything off that uses up more ram.
Finally if that doesn't work reformat your F:/ drive as you probably have everything fragments making it hard to read therefore slowing down texture rendering.
If all else fails then just run it off the disc.
If all of that fails then it is unsolvable without a new xbox.
This is how I sorted out my HD slowdown probelms in the past. Good luck.
And no I haven't played the game as it hasn't been released in the Uk yet but I'm getting friday.