TURN OFF AUTO MASKING IN CAMERA RAW
The Adjustment Brush in Camera Raw is absolutely amazing. However, if you're painting a lot with it you may notice a slight slow-down in its responsiveness. If that happens, try this. You'll see an Auto Mask button at the bottom of the Adjustment Brush panel. Make sure it's turned off (unless you absolutely need it turned on). That cuts down on the burden the tool puts on your processor and speeds the painting up a little.