Is triple-negative breast cancer a fast-growing or slow-growing type of cancer?

In general, triple-negative breast cancers are characterized by being highly proliferative: that is they are typically faster-growing breast cancers than are, say, estrogen-receptor-positive breast cancers. This may explain both why they are potentially dangerous breast cancers, but also perhaps why they are more sensitive to chemotherapy drugs that affect dividing cancer cells, so that they are in general more sensitive to chemotherapy agents than are estrogen-receptor-positive breast cancers in many cases.
When we characterize tumors, we look at the stage of the disease as well as the grade of the disease when we determine treatment. The stage of a breast cancer is determined by the size of the tumor, whether or not lymph nodes are involved with metastasis of the tumor, and whether the tumor has spread to someplace distant in the body. The grade of the tumor is a characterization of the tumor by the pathologist that allows us to determine how fast the cells are dividing. Women with a high grade, or a grade III, breast cancer may be recommended to have chemotherapy even when they are diagnosed with a very early Stage I breast cancer. Therefore we use multiple factors in determining how we treat each and every patient and not one factor alone can make these determinations. Depending on the biological behavior of the tumor, we may tailor the patient's treatment by trying to prevent a cancer from recurring not only in the breast, but also systemically in the body.