one thing you could do is install the Splunk on Splunk app, it was created by Splunk's Suport team to help them troubleshoot user issues:
http://splunk-base.splunk.com/apps/29008/sos-splunk-on-splunk
you could also review the search in the Search Job Inspector:
http://docs.splunk.com/Documentation/Splunk/5.0.2/Search/UsingtheSearchJobInspector
in particular, you can see what parts of the search are taking the most resources:
http://docs.splunk.com/Documentation/Splunk/5.0.2/Search/UsingtheSearchJobInspector#Execution_costs
however, if the search hasn't changed, it's probable that the bottleneck is elsewhere in the system--i'm betting SoS can help.
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