I have a complex Hermitian matrix of size about $70000\times 70000$. I want about 100 eigenvalues near 0. However, I know that every eigenvalues are two-fold degenerate. I found out that the running time of
eigsh is extremely slow (more than 5 times) compared to the situation with no degeneracy.
Also, I found out from the following link Computation time of eigenvalues with ARPACK depends on what? that degeneracy is bad to eigsh algorithm.
What can I do to make my code run faster? In the answer of the link suggests to increase the size of the Krylov subspace, but what size is the adequate one?