I'm having exactly the same problem.
I've updated to LR5.5 recently, and cropping has become dramatically slower, with the computer jerking around several times before catching up to where I am. The rest of LR seems to be slower since the update as well, but cropping is the most obvious and noticeable.
This is proving a serious drag on work - with thousands of images to edit as the OP said, this is colossally frustrating.
Any help Adobe?
What is the resolution of your screen?
I'm using an iMac 2013. Display information reads "27-inch (2560 x 1440)"
Only the cropping and rotating delay? How about upright or Enable profile corrections?
Would you please do a catalog optimize then try it again? (File -> Optimize Catalog) Thanks.
<Before optimising catalog>
I've tried using the upright tool and it's as snappy as ever. I've also tried manual perspective transformations, and they seem to be just as fast as they were before the upgrade to LR5.5.
I've also tried a handful of other operations (Vignetting, Post Crop Vignetting, Tone Curve Adjustment, Radial Filter), and they all respond at a decent pace. So currently, yes it's just cropping/rotating that are moving slowly.
<After optimising catalog>
I can't be sure whether I'm imagining it or not, but crop/rotate do seem to be a little smoother/faster. I was getting the "rainbow wheel" previously, and I'm not getting that now - there's just a brief pause while the image appears to re-render, and then we're underway again.
Again I might be imagining it, but it seems as though the drag/delay is greater when I'm cropping down significantly - slight downsizing of images doesn't seem to result in as much of a delay as massive downsizing by cropping.
I'll carry on like this and see whether there's a sustained improvement.
Many thanks for your help,
I have the same problem on a top specced latest version MacBook Pro.