Chrome is going to refuse to parse
document.write for users on a slow connection. On the one hand, I feel that Google intervening in this way is a bit icky, but I on the other hand, I totally support this move.
This keeps happening. Google announce a change (usually related to search) where I think “Ooh, that could be interpreted as an abuse of a monopoly position …but it’s for ver good reason so I’ll keep quiet.”
Anyway, this should serve as a good kick in the pants for bad actors (that’s you, advertisers) to update their scripts to be asynchronous.