Clearly, you've never worked in distributed systems, and your impression of Twitter (and other large "centralized" by your definition systems) are actually huge, loosely coupled decentralized systems.

The real difference is whether than decentralized, loosely coupled system is run by and owned by a single corporation, or not.

It's the design of the system itself that determines latency, not the centralized vs. decentralized argument.

You're speaking as if twitter.com is a single giant machine on a single IP address. In fact, it's thousands of loosely coupled machines and services all working in tandem across nearly the entire globe. Sounds like a typical decentralized system to me.