You can see what happens in this video: Lightroom Bug - vidme. It happens when resizing any image. It doesn't slow anything down, but it's intensely annoying. Any ideas? Thanks in advance!
The Develop module creates the screen preview "real-time." Any change you make to the image causes LR to rebuild the preview. Higher resolution monitors really need some help and I see the same lag with the Crop tool on my 2560 x 1440 monitor. LR6 is rumored to support graphics card acceleration, which may fix the preview building lag.