This phase III trial compares early treatment with venetoclax and obinutuzumab versus delayed treatment with venetoclax and obinutuzumab in patients with newly diagnosed high-risk chronic lymphocytic leukemia or small lymphocytic lymphoma. Venetoclax is in a class of medications called B-cell lymphoma-2 (BCL-2) inhibitors. It may stop the growth of cancer cells by blocking Bcl-2, a protein needed for cancer cell survival. Immunotherapy with monoclonal antibodies, such as obinutuzumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread. Starting treatment with the venetoclax and obinutuzumab early (before patients have symptoms) may have better outcomes for patients with chronic lymphocytic leukemia or small lymphocytic lymphoma compared to starting treatment with the venetoclax and obinutuzumab after patients show symptoms.