...

After a few iterations we settled on the following:

Cache keys are compound; comprised of 4 parts: <Method>|<PathPattern (or 'Path' if not defined)>|<X-Okapi-User-Id>|<X-Okapi-Token>

...

Existing Okapi (v3.1.2) and mod-authtoken (v2.5.1)
Average Check in time: 1.024 sec
Average Check out time: 1.812 sec

POC Okapi and mod-authtoken
Average Check in time: 0.729 sec
Average Check out time: 1.364 sec

...

mod-authtoken CPU utilization is 4 times better with caching. See attached screenshot and Excel file for more details

Next Steps

•  Port the token cache PoC to Okapi 4 - https://github.com/folio-org/okapi/tree/tokenCache
•  Add metrics for tracking cache events (hits/misses/etc.) - https://github.com/folio-org/okapi/tree/tokenCache
•  Re-test with this version of the PoC
•  Create stories (OKAPI/MODAT) for the implementation of this optimization - 
•  Sort out other implementation details - See Considerations above.