If you are talking about the indexer then I think you are mixing two ideas here.
The first one is being HIGH SPLUNK PERFORMANCE, that is, Splunk works fast, Splunk indexes fast, Splunk searches fast, Splunk does everything fast. If this is what you want, then you need Lots of CPU Power, Lots of RAM, FAST Disk IO and FAST network. And more importantly, you want all of the above because you want Splunk to USE all that power.
If you want splunk not to use PC resources, then do not index too much data, do not run heavy and dense searches, do not run realtime searches, do not allow more then one user at a time, do not do regex extractions while indexing etc..etc..
The idea here is, it really depends on the load you want to put on splunk. If you want Splunk to do heavy work, then you should be willing to allow it and enable it to do so with good hardware.