Non sequitur.
Your computer is doing more processing. There's WebGL, which will engage your graphics card. If it is on your CPU, the CPU will consume more power. Depending on your laptop, you could have a discrete graphics card, which will be switched to and consume more power. There are animations, etc.
It is not only because of React. In fact, by avoiding unnecessary DOM changes, it's probably an advantage (although this would have to be measured).
Unless you are telling us that you were playing fortnite or something and your fans didn't spin up, but this thing caused them to spin up.