When the Linux machine is connected to the network it can take one minute to shutdown but if it's offline it can take several minutes.
When the system shuts down, a Jobcheck runs before stop CAF to connect to the Scalability Server to see if there are pending jobs marked to "run at shutdown" that is a feature only for Unix/Linux systems.
If the machine if offline, it does several retries every minute.
CA Client Automation - All versions
Unix and Linux systems
The shutdown time can be reduced by customizing the retries done by the Jobcheck if the machine is offline with the following configuration parameters under DSM > Software Delivery > Agent: