But you can't count on two subsequent requests from the same user being handled by the same process instance.
Or the same process instance not interleaving requests from two different users.
That's not a factor. The data is lazy loaded. If process A has $configCache['whatever'] and on the very next page request on process B doesn't have that in the cache, then it will get loaded into the cache for process B, and the request will finish serving just fine.
The data is fixed config stuff. It's not changing from user to user or process to process. As each process needs it, it will create it's own global and load the data in.
-- gw