seems like as long as line width is <= 1 stroke performs slightly better (on higher pixel ratios) and provides a cleaner visual

The differences are hard to generalize across browsers and systems. E.g., for me (macOS, Chrome) fill outperforms stroke for all widths. In Safari on the other hand both perform equally well regardless of the line width. When you observe the performance graph keep in mind that other factors may also lower the frame rate (animations/processes in other tabs, garbage collection etc).