I sometimes work on my laptop until I have less than 5% battery remaining. At this point, the laptop is sluggish because it’s using every last bit of power to keep the machine on. It doesn’t care about app speed. It’s in pure survival mode.
If I want proper performance, I need to plug it in.
If I don’t want my laptop to ever be sluggish, I need to charge before it’s necessary.
It works for laptops, as well as humans.