Autoplay on Facebook and Twitter is distractive eye candy. How can you avoid looking at something that's moving as soon as you log-in? It's already bad enough that every time I log into Facebook I forget why I went there in the first place.
The internet hasn't gotten any faster (at least where I live) but the UI design has. These networks are competing for eyeballs. They'll make ads move faster to guarantee advertisers more impressions.
But as a user these conditions make me second guess logging in. The Internet is TV, for real this time. Tumblr TV is already a thing.
They say you lose the ability to multitask with age. Having videos and GIFs move as I shift through the feeds is like having little birds flying by as you walk.
There's no way to switching off autoplay and reverting back to static opt-in. The social networks are making attention impossibly sticky. What's the Internet's version of cord cutting?