Only on a truly ancient system. On anything faster than about 200 MHz, decompressing pictures on a webpage should be trivial. On a 100-200 MHz system, it should only be noticeable if you are simultaneously decoding 5 or more pictures. And most web browsers limit simultaneous downloads, so you won't often be decoding that many.

"Turbo" modes are great for limited bandwidth (dial-up,) or EXTREMELY limited CPU (mobile devices like a Crackberry.) Essentially useless when you have a desktop/laptop with high-speed internet. On most modern web browsers, especially with slower systems, the page layout is the slow part. Turn off CSS and Javascript, and even the seemingly simplistic google.com renders faster; even though the picture is the same.