A method includes converting a first threshold voltage related (VT-related) cell of a first standard cell library to a first modified VT-related cell. The first standard cell library includes the first VT-related cell and a first base cell. The first VT-related cell and the first base cell each include different portions of a layout design of a first standard cell corresponding to a first performance setting. The method includes generating a second standard cell library based on the first base cell and the first modified VT-related cell. The first modified VT-related cell and the first base cell each include different portions of a layout design of a second standard cell corresponding to a second performance setting. The method further includes generating a layout design for an integrated circuit based on the second standard cell library; and forming a set of masks based on the layout design.