See that “Write” graph that shows constant 600MB/s+? That’s your storage device installing/updating whatever you are downloading. It does not download everything and then installs it, it downloads in “chunks” and then installs/updates in parts. So the download will resume as soon as the part downloaded earlier finishes installing/updating. That’s the expected behavior, nothing wrong with your internet.

More information from the OP would be helpful. I assume that you have gigabit internet? Is it fiber, and do you have both an ISP router and your own router? Or just an ISP router? Are your speed test results from wired internet, or wifi? Given the 1 second latency, I’m guessing that’s a wired speed test, but just a guess. Really, more information about the OP’s network configuration would be REALLY useful to be able to make helpful comments. The mfr. and model of the big pieces is always helpful as well. I can think of some causes, but they depend on your configuration relative to your ISP plus your home network infrastructure.

Check your disk usage, probably maxing the write/read speed.