Block time or delay is often a problem in Bitcoin. Then would it be possible to develop a decentralized cryto currency with zero delay/block time? Any reference will be appreciated.
A delay of some kind is unavoidable.
Say you see some transaction, say Alice sending a unit to Bob. Before you can accept that transaction as reliable, you need to be sure that everyone will not eventually agree that Alice sent that unit to Charlie first.
Alice can compose a transaction sending to Bob and a transaction sending that same unit to Charlie. She can send half of the nodes one transaction and half of the nodes the other. It will always take some time for the system to agree on which of those two transactions to execute and to assure itself that there isn't some third transaction, also sending that same unit, that won't be agreed on.
That agreement will necessarily take some time as nodes have to talk to each other. You can avoid it by having a designated master whose word is law, but it's not clear how that could be decentralized. Among other problems, you need to detect the failure of the master and change the master. Which means you need to make sure that others know the master didn't fail -- because just because you think the master is working doesn't mean the master's messages are getting to others.
You also need to protect against the master telling half the nodes to accept the transaction to Charlie and half the nodes to accept the transaction to Bob. If they don't make sure the master is honest, then the system is, again, not decentralized.