True Tom, I did not test concurrent accesses to the cache I don't know how that performs. I guess you'd never have more than N threads number of concurrent acceses and at 9 microseconds per access, I don't see too much of a locking issue on that. It takes alot longer to write to util_memoize cache.
I just tested and it looks like it takes around 45 microseconds to add an entry to the cache if its not in there. So its still very fast to fill the cache and you are doing that incrementally not 1,000,000 entries all in one go.
