From SU01 we can specify default printer to the user but do not check the box "delete the request after output immediately" which improves the spool performance.
The printer can be locked during maintenance in SPAD
To process the requests sequentially based on serial numbers
Select the option -- [ ] print sequentially in SPAD from OP devices attributes tab.
Print sequentially consumes time to print in the order. If this is unchecked it prints faster but sequence is not maintained.
You can also get information regarding SAP BASIS