The ResourceManagerImpl.getResource() method is synchronized, which makes it difficult to share a Velocity Runtime between threads in an environment such as a j2ee web application.
After upgrading Velocity to version 1.5 in Roller and running some performance tests I saw a very noticeable decrease in throughput for the application. I fired up jconsole and noticed that almost all of my app server threads were in a BLOCKED state and were waiting on the ResourceManagerImpl.getResource() method.
In my particular case the difference resulted in a loss of 2/3 of my original ops/sec, which is pretty huge. After simply switching Velocity back to the 1.4 release and rerunning the test I saw the results I expected.
I assume this is overactive use of Java synchronization because the developer guide suggests that the singleton model is "very appropriate model for use in a Servlet 2.2+ compliant web application".