As Ben mentioned last year, creating a large fixed size VHD in Hyper-V can be a lengthy operation as we zero out the contents of the file during the creation process. In Hyper-V itself, there is no workaround to this.
However Chris, a developer on our team, has just posted up a utility called VHDTool which can create large fixed VHDs in seconds. If you are comfortable with the security implications of using such a tool and want large fixed VHDs created quickly, it could be an option for you. The utility is available with source code and can be downloaded here.