The more technology that a device has installed on it and the more powerful that technology, the longer it takes to restart the device. The restart time is the time it takes a device which is in a state of "forced offline" to reach a state of "online unassigned" once you have clicked on the restart link.
The lowest possible restart time is 54 seconds which is the time it takes a device with no technology installed on it to restart.
To calculate the restart time for a device, the system adds up the effect on restart time of every technology installed on the device. You can see what the effect on restart time for a particular technology is by looking at the technology details page (click on the specific technology on the technology tab, or search for it by typing its three letter abbreviation in the search box).