A method of replacing standard cells with high speed cells in the design of a circuit using a computer program, said application specific integrated circuit design comprising a plurality of high speed cells and a plurality of standard cells, said high speed cells and standard cells being arranged to form a plurality of paths on said application specific integrated circuit, said method comprising the steps of: timing said plurality of paths identifying cells occurring on paths for which timing targets are not met; upgrading at least one of said identified cells to a high speed cell.