Create a asynchronous - singletone WCF service

Create a asynchronous - singletone WCF service

What is the best way to create a WCF service that will work for a number
of clients but will provide equivalent information stored on the Service?
I tried the following way:
[ServiceBehavior (ConcurrencyMode = ConcurrencyMode.Multiple,
InstanceContextMode = InstanceContextMode.Single)]
But because service it singletone it works very slow (probably it
synchronous) when there are more than three clients.
But I need that service will be singletone, because I have an object that
suppose to gave the same information to all clients (or there is another
way to do this?)
I would love to have other offers.