This question is really meant for the developers and not meant to look for a work around of any kind.
Already in Develop mode. May have already made other changes (it matters not). Hit the Crop tool and the computer sits there and waits for a second or two before becoming active. And attempts to begin changing the crop (after the tool becomes active) are met with frustration for the first moments as the UI does not respond.
Lightroom 4.1 final release, running on Win7-64 with a Core-i7 processor with 8Gb ram with all images, catalog and previews sitting on an SSD drive. Images are from either the 5D Mark II or 5D Mark III.
It just has me baffled. Lightroom has already rendered the image (whether standard size or 1:1 it matters not). It's already sitting in develop mode (no mode switching here, just activation of the Crop Tool).
Running at Core-i7 speeds, a second or two is an "eternity" with the billions of calculations the processor could be performing. So what is it doing while waiting to activate the Crop Tool?
Lightroom 3.x (any version) was fast at activating the Crop Tool. Lightroom 4.x (any version) is slow. What's the difference? What is 4.x doing that 3.x did not do before the crop tool becomes active?
There are many other issues. This is just one. Let's start by speeding up the Crop Tool ... k? It's just such a simple operation, it's embarassing.
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?