Internet becomes good during speed test but terrible afterwards

Video of my problem: https://youtu.be/k37piC9vP60

Hello,

My internet connection generally works fine, but my in-game ping has been terrible lately. I ran several speed tests and noticed something strange: whenever the download speed portion of the test starts, my ping immediately stabilizes and stays low. As soon as the download test finishes, my ping spikes again and becomes almost unplayable. Edit: I forgot to mention I am using ethernet.

Does anyone know what could be causing this?

Things I've already tried:

• Restarting my PC
• Flushing my DNS cache
• Changing my DNS server