AO3, like all of the internet now it seems, uses a service called CloudFlare. Which I don't blame them for--any and every public site can become the target of a DDoS at any time for no known reason. And it has happened to them.
CF is, fundamentally, a big business service specifically to block automated access to web sites. FFF is, fundamentally, a way to automate access to web sites.
We've been dealing with this for years now on fanfiction.net and I ran out of energy to fight it directly long ago.
AO3 has publicly reported that they are having server problems and using a higher level of CF blocking. Hopefully things will work better again next month.
In the meantime, there's the
Browser Cache feature, also discussed here repeatedly.
There is another possibility I'll post about separately.