Well for a start you need to know how they got into your system, sure you could patch it all up in '300ms', but they'd just hack you again straight away.
Sure, but that applies to any kind of malware and is orthogonal to the actual question of ransomware. Fixing the vulnerability is more often than not simply an update of the vulnerable server software, while recovering the data and bringing the entire IT infrastructure into a functional state is currently the time consuming part.