Since you're limited to the third-generation CPUs (nothing newer will work), and since that's the fastest one that was made, there is no upgrade CPU you can use in that Alienware notebook.    
Hi All - so I think I'm about to buy a M18X R2 in very nice condition, fully loaded with 32GB ram, i7 3940XM, etc. I realize it's old and this is Aug2021 but... it still seems this machine is more powerful than the Latitude E6520 I've been using for Ableton & music production applications.  My main question is - is it possible (and worth it) to upgrade the CPU?? If so, any recommendations? Thank you in advance.
M18x R2, purchase, CPU upgrade advice?