"It's like the designers think you have enough knowledge as a user to know which is good to turn off, and which would be a disaster for you: so you get hassled with that sort of nonsense from time to time so they can smugly think of the pain they inflict on us all and presumably laugh into their cappuccinos. I just close the warning window and ignore it."
It's not that though... It's because users often install browser plugins without realising/thinking (such as AVG safe search). The browser knows when it's going slow because of a third party plug-in, and lets you know things could be quicker, if you just tidied up a bit :)