why does my internet usage fluctuate like this?

Like the title says, I'm confused about the behavior of my computer's internet usage when downloading games, updates, etc. Why does it fluctuate between zero mb/s to over 100mb/s? A possibly related issue I experience when downloading things is that my pc runs like 10 times slower, but I don't know if that is connected to my issues with the internet speed. Is there any way to fix this? I can add specs if needed.

(I have tried to research this on my own but i haven't been able to gather much so I decided to ask here instead)