We support low-end devices well (although this is a constant balancing act)! Bevy runs well on iOS currently (and we have WIP Android support ... we're working on re-enabling that asap). Fans spinning is a known issue, but it is generally decoupled from framerates (which tend to be fine). On some machines we are over-engaging cores, even with empty apps that do nothing ... my current theory is that we need to adjust how our task pools are managed.
iPhones have insanely performant GPUs for the form factor. I wouldn’t be surprised if the newer ones get better performance in Bevy than a lot of perfectly usable laptops.