Rational Publish Engine (RPE) - Java API [How to increase the heap space] to generate document faster

We are using ArtifactDocGen.jar to generate pdf of DNG modules

i have sent Xmx value while calling the jar but the document generation is not fast enough.

Atleast for one artifact it takes around 20 seconds..

I believe the Xmx value set during jar call is not helping here

Internally it calls the RPE.exe where to set exactly the Xmx value if i call via 
ArtifactDocGen.jar

I know .ini files we can set -> but no effect.

sample code:
IDocumentgenerator generator = new LocalDocumentGenerator( new RPEEngine());
EngineStatus result = generator.publishSynch(docspec, 0);

Someone please help here

Not able to set the heap size when generating document via 
ArtifactDocGen.jar