What could be the causes for System performance is slow and where to check, resolve SAP

1. Might be issue with resources availability. Some times there would be not enough or insufficient work process resources available during the program execution. 
2. It can also be issue with the Memory consumption and CPU utilization.
3. Network connectivity issues are one of causes for SAP system slow performance.

In the above case, Please follow the below procedure and resolve the issue:

SM66: To check overall load on all instances across the SAP system.
SM50: To check work process load on specific instance, here identify the associated work process.
SM37: Check for the long running jobs
SM12: Check for lock list held for long time
ST22: Check for any connection time out dumps occurred.
SM21: Check for error message logs related to not enough resources
ST03: Check for response time of transaction code
SMLG: Check for the load distribution (F5)
ST02: Check for memory/buffer with more usage and overall swaps as well (mostly red in color)
ST06: Check for CPU utilization (should be above 90% idle)
Check for any backup is running or not. It may affect to SAP System Performance if backups are running.