My Retina MBP had been perfect. Opens stuff up fast. My ISP speeds are around 50 down and 10 up. I usually use Chrome as my browser. Lately, I have been getting very slow in opening up different URL's, sometimes up to 10-12 seconds, where it had been 1-2 seconds. At the bottom of the left page, I would see the "resolving host" posted until the URL finally arrived. After trying various things suggested elsewhere, I unplugged my modem and my Airport extreme router. I waited about 30 seconds, then plugged my modem back in and waited until it had stabilized, then plugged my Airport Extreme back in. Voila, fixed.