Ok, I found the my bug and got a feature request for plantuml.
1) slow generation and bloated diagram
During my try and error to get rid of the java.lang.UnsupportedOperationException: PDF error I didn't use our DiagramStylesheet. I commented it out and forgot that there was an option skinparam componentStyle uml2 on which the test diagram heavly relies on. Having this option enabled the diagram size and compilation speed get back to normal.
2) cryptic expection
But where comes the exception from? This took my some time... far too much time! As mentioned above we use a custom Stylesheet with all our skinparams we need for diagram generation. I forgot to put this in the repository so the plantuml run failed.
Could you please add a verification step that looks after all included files (!include xyz) before starting the compilation run? An useful error message would be really nice, in case that the included file(s) cannot be found.
Thx & bye,