SAP Kernel related

http://www.guru99.com/sap-kernel.html

SAP CRM Administration and Performance Tuning - BASIS Consultants

http://wiki.scn.sap.com/wiki/display/stage/SAP+CRM+Administration+and+Performance+Tuning+-+BASIS+Consultants

SAP Performance tuning related details

https://soumen.wordpress.com/performance-tuning-of-sap-another-experiance/sap-tuning-gather-symptomps/