Spread Toolkit is a C++ library for messaging and only has partial support for JMS. It doesn't support durable messaging, transactions, XA or full JMS 1.1. It is also dependent on a native code Spread daemon running on the machine.
Apache ActiveMQ on the other hand is the JMS provider used in Apache Geronimo and is J2EE 1.4 certified in Geronimo and is 100% pure Java. ActiveMQ supports transient and durable messaging, transactions, XA, J2EE 1.4, JMS 1.1, JCA 1.5 as well as heaps of different features like Message Groups and Clustering
If you're not convinced by performance reports then please do try running performance tests yourself. You might wanna check out our overview of Performance or try using out the ActiveMQ Performance Module Users Manual
The Commercial Providers on the Support page may also be able to help diagnose performance issues, suggest changes, etc...