The "adjustment brush" in ACR 8.4 has a tremendous lag in PS and Br, is OK in LR. Everything was fine for me in ACR 8.3. MacPro 3.33 GHz 6 core Xeon, 32 GB ram, Rad ATI HD 5870 graphic card, (No difference in basic or advanced Processor setting). OS 10.8.5. All the other ACR sliders work fine. This condition was also present in the 8.4 beta version, and was fine when i switched back to 8.3. I believe this is a core issue with 8.4 because it is also present in my other computer.