The question of what is the fasted way to force a reboot when not having to flush the buffers is something I've resolved for myself in the following ways:
"real" hardware (e.g. a notebook running on mains supply, this is to not needlessly wear down the battery by constant charge and discharge cycles): I just literately "pull the plug" (i.e. disconnection of the power supply for a fraction of a second is enough). The "v2.0" of this approach is an additional switch I've put into the mains cable that runs into the AC adaptor, as this switch can be placed into a more easy to reach position.
VirtualBox VMs: The (virtual) "Reset button" is the 'Host Key - R' keyboard short cut (with a default host key to be the right 'Ctrl' key).
QEMU VMs: Changing to the QEMU monitor (with the 'Ctrl-Alt-2' keyboard short cut), and then issuing a 'system_reset' command. Whilst that might not look like much of a savings compared to a 'sudo reboot -f', but as one can use the command completion feature of the QEMU monitor I just have to type "s y tab r tab" (i.e. five keystrokes), plus if I have to repeat this reset I can easily use the history function of the QEMU monitor (e.g. via the Up key).