Basically, decentralization eliminates a middleman and excessive fees. An example would be: you send your Bitcoin from you to your cousin in California that's it. No middleman, minimal fees, if not free!
Lightning fast you're talking, 3 to 5 seconds. Bitcoin, as well as the rest of cryptocurrencies are decentralized. Blockchains are politically decentralized meaning no one controls them, and architectural decentralized meaning no effort structural central point of failure, however they are logically centralized meaning there they act like a single computer but they're not.
Bitcoin was designed as a decentralized form of exchange to the regular fiat currency so there's no single point of failure which makes it stronger more efficient and Democratic it is based on the blockchain which allows for it to be decentralized is a single user program to another single user. No Central Bank involvement no government involvement is just straight peer-to-peer. .