"'wanna cry' encrypt our data within a minute. So how can they do it
that much faster speed on network?? "
It encrypts attached network drives so the speed will be down to the CPU of the machines and network bandwidth. Also the size of data to be encrypted.
Also remember should other network machines be vulnerable to the SMB flaw then the worm is installed on that machine - meaning the contents of that encryption is local to the infect machine (not done over the network from the original machine).
Is my file encrypted by "wanna cry" then any way to decrypt my file
without sending bitcoins to hacker(s)??
In short no. But if you have not turned off your machine you could try looking at an approach detailed in this article from wired: https://www.wired.com/2017/05/wannacry-flaw-help-windows-xp-victims-get-files-back/
(This attmepts to recover the two prime numbers used for the public/private key pair generation)