The vacuum thickness is also a crucial parameter affecting the computing cost and speed. Evidently, if the lengths of unit cell vectors along x and y are quite large, each step during the self-consistent calculation will need more time.
i) track each step of the self-consistent calculation, and find out the average time for a step as well as the change of charge in each step;
ii) check the memory consumption for each node during the self-consistent calculation, and see whether the memory was exhausted or not.
If the change of charge is dramatically fluctuant, the slow convergence may be caused by the parameters specified in the input file and related to the charge mixing and the determination of occupation.