I have a brand new Mac Pro 6,1 (6-cores, 32GB RAM, D500) and I'm having irritating problems with Lightroom 5.5 (CC). Internal Storage Lightroom Library (approx. 90-110k images) file Lightroom Previews file Lightroom Smart Previews file USB 3 Storage (7200RPM 3TB) RAW files (CR2, DNG mix) I have found that the adjustment brush is not very responsive. I will paint on adjustments showing the red mask overlay and see this blocking as I brush like the computer is really struggling to keep up with my strokes. Once I'm satisfied (at last) with adjustments I'll copy the adjustments, switch to the Library module, and then paste the changes to 200 or so similar images. This is where things get really irritating. Once the adjustments are pasted I'll double click an image at random to confirm that everything looks the way I want it to. Lightroom takes up to twenty seconds to render the image in the Develop module. I went back to the Library module, selected the mass of images, and instructed Lightroom to render 1:1 previews. These previews take 20-30 seconds to render per image. I'm working with RAW files from a Canon 6D (22MP ~ 25MB each). WHAT is going on here? I'm not sure what I can do to make things snappier. Smaller library? I can see that the processors are in use to the tune of about 500% during the render process and LR is using only a few GB of RAM.