If for whatever reason your upload gets saturated it will trash your download rate as well. I used to see that all the time with Bittorrent clients. When seeding you had to leave yourself some overheard on upload, and don’t let the upload stream hit capacity as it doesn’t have enough bandwidth to manage the downloads. I’ve never run a bandwidth test with MSFS running so I’m kind of curious to try that now!
It looks like you just had the sim open with no flight loaded, but it wasn’t clear whether you were downloading any updates as the top of window indicators were covered by the speed test results.