Question: Is it possible to only display particular category (e.g. 'homepage') posts on the Tumblr frontpage?

The other categories I want to be available using a 'redirect' page which.

Answer: 
I figured it out myself.
I still don't know the exact reason, but somehow the problem is caused by performing this code multithreaded and not by loading the ontology itself. When I run the code without multithreading, the loading only takes several seconds
If you are seeing the problem only when running the code in a multithreaded environment, there is a chance you stumbled upon some OWL API bugs in that respect, in the past this has been caused by HashMaps used as caches in various places and causing threads to get stuck if a change to the map happens at the same time as a read access.

200 classes is a fairly small ontology, so it should take seconds. If you can reproduce this situation consistently, please report it as a bug on github or on the OWLAPI mailing list.