Using 10 threads is still not fixing it for me... is there something particularly weird with the Skyrim SSE Legacy of the Dragonborn mod? My download speed (even at 1 thread) would shoot up to download all the other mods I've decided I want, but for this mod in particular, even after going premium, bumping up to 10 threads, re-starting the download from scratch, and restarting vortex multiple times, I'm consistently not managing to hit even 50 KB/sec.
When you switched Vortex to use 10 threads, did you log out of Vortex, restart it and log back in?
Because just switching it and expecting your next download to be fast probably isn't going to work.