The top four web performance challenges
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At the top of Jeremy’s list, other people’s Javascript:

At number one with a bullet, it’s all the crap that someone else tells us to put on our websites. Analytics. Ads. Trackers. Beacons. “It’s just one little script”, they say. And then that one little script calls in another, and another, and another.

It’s so disheartening when you’ve devoted your time and energy into your web font loading strategy, and optimising your images, and unbundling your JavaScript …only to have someone else’s JavaScript just shit all over your nice performance budget.