You absolutely can have sub-second responses to search queries for publically accessible decentralized content, if it is indexed by a regular search engine. That is what the web is.

Privately accessible content tends to be much smaller so local caching works. And if communication is not meant to be instant, email proves that a decentralized system works.

What gets a little more difficult is content that is widely available but behind login. Facebook has this but twitter and mastodon tend to not be this way.

In fact mastodon is a workable system for decentralized social networking, so I think the skepticism is unwarranted.

The only question in my mind is whether a popular social network can be effectively bootstrapped without a core capital incentive.