I use a quad-core MacPro with 16GB RAM and ATI Radeon HD 5770 graphics card with dual 27-inch Apple monitors. when using LR 4.1 histogram and preview updates are very quick until I enable the second monitor. Then it takes 10-15 seconds to calculate the histogram and previews. If I then disable the second display it still takes 10-15 seconds to calculate the histogram and previews. Only disabling the second display and restarting LR4 returns to normal. This slow performance makes the second display unusable for all practical purposes.