Daily volume less than 800MB is supposed to be small enviornment. In genral, splunk need high disk io to get search result faster. As for Xeon L5609, clock size 1.86 GHz is not fast. Are you using recommened server suggested by splunk? Please see as bellow.
http://docs.splunk.com/Documentation/Splunk/latest/installation/capacityplanningforalargersplunkdeployment
In typical, you do not need second splunk instance when daily volume is less than 100GB. In your case, you may need to upgrade your hardware spec CPU and DISK in order to make search faster. Please verify your environmnet.
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