1- Make sure you aren't overheating.
2- You can use "Texture" instead of "RAM" (EFB Copies). You'll have to modify .ini file (User/GameConfig). Use "RAM" only when needed.
3- If it still slow then there's nothing you can really do. Your CPU is just too weak. You might wanna try some older versions of the emulator (faster but more buggy and not supported on the forum)
Your CPU is slow for this game...