I am having a problem with LR4 processing images excrusiatingly slow. Changing from one image to the next gives the whirling rainbow of death for as much as 30 seconds. Highlighting an image in Library and then clicking on Develop gives an empty LR screen for 30 seconds or longer, then the image appears in Develop. Every click or drag with the brush tool also gives the whirling rainbow of death for 10 to 20 seconds, and so on. Print module, same thing. I thought it might be the relatively new Seagate external hard drive I was using, so I moved the entire folder of images (3300 images) to a hard drive that I have been using for years. Same thing. So then I moved the entire folder of images right onto the Mac under Pictures, to make certain it isn't the HD (I don't normally keep images on the 1TB internal Mac drive; I am only using 1.6GB of space with all my programs and have 10GB RAM). Same problem. Photoshop, Word, Outlook, Acrobat, et al, are working normally. The images only bog down in LR4. Never had this problem with earlier versions. Anyone else experiencing this?
Would you please be a guinea pig and try this: delete all the previews from your Lightroom folder(s). Then open Lightroom 4, select the images in your catalog and generate new standard previews. This worked for me and some others when we had the same problems you have described with Lr1.1 and Lr3.0. It can be a lengthy process.