IE6/7 launch times were much faster than Firefox (this was important before SSDs were common) and general performance was better on IE. I used Firefox for about 15 years mostly because of it's ad-blocking capabilities, even though it was generally a slower browser than IE. IE was "slow" only after Chrome became a thing... which was faster than IE. Firefox wasn't.
Having said that, in this context, these things don't matter at all. You can make a barebones browser which only supports HTML and CSS and get several fold improvements in battery life and performance. That is why IE was bad. It did not support newer better functionality that other browsers did.
Having said that, in this context, these things don't matter at all. You can make a barebones browser which only supports HTML and CSS and get several fold improvements in battery life and performance. That is why IE was bad. It did not support newer better functionality that other browsers did.