Question state caching
The current cache is very close indeed to the CAS. Indeed, it is actually calls to the CAS itself which are cached. This gives a dramatic increase in performance when the result already exists.
We recommend using the cache on a production server. Note, the cache needs to be cleared manually, perhaps on an annual cycle. The cache is automatically cleared if you upgrade the plugin.
Clearing the cache
This can be done through Moodle.
Home > Site administration > Plugins > Question types > STACK
Choose to view the healthcheck page. At the bottom of this is a button to clear the cache.