It's pretty easy to do, but a little involved until you get used to the procedure. See https://docs.splunk.com/Documentation/Splunk/latest/Data/Configureindex-timefieldextraction for instructions.
Yes, it will increase indexing time. How much depends on how many fields are extracted and how complex the field extractions are. One simple field should have a minimal effect on index time.
---
If this reply helps you, Karma would be appreciated.
