When multiple threads (requests) accesses the Singleton, the above constructs ensure that the state of the object is consistent between subsequent accesses. The access sequence is determined by the request processing sequence of the server in a multi-threaded environment (mostly FIFO).

As you can see, introducing explict thread-safety constructs are expensive. Typically, a Singleton object used for read-only access (for example to cache data, provide lookup values, storage for system variables set at startup, etc.) are the most optimal implementation of the Singleton pattern.