I have two co-relation search in ITSI which is scheduled to 15 minutes of windows, what is the appropriate scheduled should be set to avoid any performance issue in future.
There are no definitive answers, it depends on your search time, the server concurrency load, and the desired time to be notified of alerts or the frequency.
Example : you run 2 correlation searches running every 15 minutes, looking back at 15 minutes.
If you have an alert condition happening t=0, then the correlation search runs as t=10m, then it will take you 10 minutes to be get a notable alert created, and a few seconds to get grouped in an Episode and trigger an action.
Changing the frequency will help control the time before you are notified, it may vary based on the type of data you are monitoring,