In MultiThreadedConstructionHeuristicDecider and MultiThreadedLocalSearchDecider there is this code:

// Tell the move thread runners to stop
        DestroyOperation<Solution_> destroyOperation = new DestroyOperation<>();
        for (int i = 0; i < moveThreadCount; i++) {
            operationQueue.add(destroyOperation);
        }

That could probably first do this, to improve shutdown performance:

operationQueue.clear();

That will clear the non-started MoveOperations. It will also clear the non-started StepOperations. Because this is a poison-pill, that's fine.