I think one big issue is that there are OS specific APIs on Windows and Mac that allow you to only redraw the pixels/layers of the web browser window that have changed, with the unchanged portion retained in memory and recomposited.
We've seen versions of Safari and Edge that leverage those sorts of APIs to deliver better battery life on their respective platforms, but if you are writing a cross platform browser you may not be willing to do extra work for each individual platform.
Several years ago, Firefox adopted the solution Chrome used of dividing the web browser window into large-ish sections, so you could at least skip redrawing sections that had not changed, but that still leaves you doing unnecessary work, just less of it.
Prior to that, the battery drain using Firefox vs Safari on newer machines with higher resolution displays was very noticeable.