malignant cells take just as long as normal cells to divide, but they do not go through any S phase(or a very short phase). normal cells need growth factors in order to divide, but even so i don’t know if they do it as fast as the cancer cells.
Actually non-transformed cells such as fibroblasts grow faster than transformed cells. If you’re growing a cell line from patient material this is your main concern that it might get overrun with normal cells. Cancer cells form tumors because these cells do not go into apoptosis pushing the total balance towards growth albeit slower.