Target block time is 5 minutes.
If I understand it correctly, that means every transaction need at least 5 min to complete. I think it's too long for a efficient system where people could do their bitassets trading business on it.. Any idea to reduce the transaction time.? I know it's difficutly since ripple way does not work as your assessment.
In a decentralized system you are limited by the speed of light if you want stability. Strictly speaking blockchains do not need to be decentralized to serve their purpose. Someone could implement the exact same transaction model on a central server and as long as the blockchain was public record you could trust them and if they were shutdown someone could just migrate the chain to a new server. Such a chain could offer instant confirmations and still be trust-free, but centralized.
Thus I content that decentralization has costs but is not necessary for the trust-free revolution. For now people expect things to be fully decentralized because of regulatory issues.
Ripple consensus is ultimately a fig leaf of decentralization that is only slightly better than having a dedicated central server with automatic fail over. The purpose of consensus is to agree on the next closed ledger so I can see it working something like this:
Everyone agrees to let Sam decide on the next closed ledger and he can have that role until he abuses it even once or he dies. Then we have a chain of command where if Sam is no longer able to perform the job (arrested, etc) then Alice takes over. Alice could even be set up behind a TOR hidden service. Neither Sam nor Alice could generate alternative chains without getting caught their alternative chains would be rejected. Furthermore, Alice could simply sign everything Sam produces and therefore they would have to collude to produce an alternative chain.
I think you can see from this that decentralization of block production is more a necessity of marketing than of security of the network. Though some could argue that it is a necessity of security for those that would end up signing every block as they would become giant targets for government shutdown. In the future, once governments lose this war against crypto, then many centralized services will compete to offer instant transactions based upon the premise of open/audit-able blockchains.