Why Not Blackbird?
First, Blackbird is meant for PHP developers. We're not saying "Java developers need not apply," but if you prefer Java over PHP, Blackbird isn't your best choice.
Blackbird is pretty efficient. As much as possible, it relies on native PHP APIs for speed, and it itself doesn't use much memory. It will never be as fast as a compiled language, though. If you need speed, the Zend Optimizer may help, but we haven't yet tested this configuration. If you expect to be processing millions of transactions per second, and you need millisecond response times, Blackbird is probably not for you.
Finally, Blackbird was written to solve a specific type of problem. It's not as functional as a full-blown ESB, such as Apache ServiceMix. For instance, it doesn't use XML service endpoint configuration files packaged with a component. This actually makes it much easier to deploy a component, especially more than once, by simply adding a second configuration file that references the same class. However, it makes it harder to manage the architecture in a large environment with hundreds of objects. Also, it was primarily intended for creating modules that can communicate well via fully decoupled, publish/subscribe broadcast models, rather than transaction-oriented queues. If you need tighter coupling or support for transactions, consider a Java ESB such as Apache ServiceMix.