The first of these would only be useful if you need to apply EIGS to many problems in parallel, in which case each of these could be solved in a thread.
The second will usually make sense if you have a parallel cluster over which the matrix could be distributed, which I'd expect only makes sense for a larger matrix than 16384-by-16384.
EIGS isn't always running single-threaded, it depends quite a lot on the problem being passed in. How much time is EIGS taking to solve your problem? Could you run your EIGS command with the "Display" option set to true, and post the results here?