Hi! I've got Lightroom 5.5 on my Macbook Pro Retina 2013 late. Just found one strange thing. Usually I work with big external display. Now I need to use just laptop screen and as it quite small, I decided to hide left side panel in develop mode. When I do it LR starts work very very slow. The most visible in crop function. About 1 minuses of waiting for result. But when I show left panel back LR becomes fast.
Just tried to do the same (hiding left panel, cropping a picture) without any impact on speed.