Disabling pre-fetching is to prevent a TCP connection to be established at all, and this is something that needs to be also dealt with in Firefox -- it's not just a Chromium thing. This ensures that no TCP connection is established before it has been decided whether a network request must be blocked or not.
Chromium used to not be able to support the pre-emptive blocking of network requests, but that has been remediated in 2012 with its webRequest API in Chrome 17.
