5/5 in terms of speed. I boiled a large mug of water in 53 seconds. I like that theres no minimum amount of water needed either so you can actually just boil what you need. I'm not sure how well it will retain heat if you're someone who boils once and then gets a few cups out of it over time. It looks beautiful at first but after being used you will get condensation all down the glass giving it a less clean look. Having to take the lid off to fill takes some getting used to as adding an extra step opposed to a flip lid. These are the only things I don't like about it, nothing major really