Modifying transform and opacity triggers the least expensive rendering path because: 1) These properties only require composition, bypassing layout and paint operations, 2) Can be hardware-accelerated using the GPU, 3) Don't affect document flow or other elements' positions, 4) Are optimized for animations. While other properties like height, width, margin, padding, top, and left trigger layout recalculation (reflow) and often painting operations, transform and opacity changes can be handled directly by the GPU on a separate thread, resulting in better performance.