Been using LR since inception and have never experienced the are having. When I click the crop tool it is active immediately. I have a very modest system in comparison to yours, Win 7 home premium, 4GB ram. Actually it is the first time I have seen this complaint posted. Just my experience and I am sure it is frustrating for you. I am not a Canon 5D MK3 user.
Wow. For me on a very similar Win 7 machine (I lack the SSD), entering and exiting crop mode happens with no discernable lag. That's with 12 MP images.
I don't think that your problem is with LR, but rather with how your system is set up. What else is running? Antivirus?