This is related to the "testPerformance" unit test. I had already considered to deactivate this test because performance testing is not important to be done during unit test. However this test does also very nicely validate that the the filtered iterators returned by the IndexedGraph implementation return the same triples as the SimpleMGraph implementation used as a reference.
Such tests are also included in the default test set provided by Clerezza, but in this case they are performed on a very small set of triples and therefore may parse by accident. This tests uses currently a Graph with 100k triples and a randomly generated set of iterations what makes it very unlikely to miss issues in the implementation of the Comparators used for the internal indexes.
Because of that I suggest that you adapt the following parameters of the "testPerformance" until you reach an acceptable execution time:
int iterations = 1000;
int graphsize = 100000;
TestCase testCase = new TestCase(sg, 100, 5, 100);
You can safely change all parameters. However I would not reduce the third parameter ('5') to TestCase.