1st easiest thing to start with is to download and install the S.o.S app (app link here) If you install this on your search head, remember to deploy the TA (Links here on the documentation tab) to your indexer(s).
In the S.o.S. app, check out the "Estimated percentage of total CPU used per Splunk processor" panel under the "Indexing Performance" dashboard. This will let you view where most of your CPU processing time is going. most typically it is a bad regex.
Then it is a matter of finding the bad regex that was put in place, through exploring your transorms.conf settings through the S.o.S. "Configuration File Viewer" view.
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