Can I make maven-assembly-plugin go faster?

Try increasing memory by specifying MAVEN_OPTS=-Xmx[some large value]. I managed to speed it up significantly, but it is still slower than tar. If I have time I will get the sources and profile it.

I've seen this happen when the maven reactor is huge, for a multi-module, multilevel project with enormous dependency hierarchy. If this is the your case, you might consider splitting your maven project to smaller ones, or suggest/ask for improvement for the maven assembly-plugin.