Sounds like you already know field extraction is the issue, and which search is slow and have performance issue. And, you would like to know which field extraction is causing performance issue for the search.
Most likely log you get help is search.log in your search artifact. So, enabling DEBUG log ( $SPLUNK_HOME/etc/log-searchprocess.cfg) might help. But, usually reading through the debug log cannot really help except for you see some error or warn message. So, identifying which field extraction is causing performance issue is usually difficult to be identified by a log file.
I would try to break it down to pieces and see when the performance issue happens.
If you're not sure if field extraction is causing the search performance issue, this troubleshooting would be more complicated, indexers, buckets, event volume, memory, search commands types, etc, etc.
You could use the job inspection to examine your search.
You can find it under the Process button.
Everyway I suggest to verfy disk i/o that should be at least 800 iops, usually this is the Splunk performances problem, I used bonnie++.
Thank you. Yes, I use the job inspector often. I was more interested in identification of bad extractions for searches that others are running in the hopes of mitigating issues before they affect other users. Like maybe something in splunkd.log that I could use to alert us when a bad extraction is negatively impacting search performance.