That's not an SSD, it's a HDD. That explains everything! It needs to write several TB of data, read a lot of data, and have random access all the time. My SSD was already the limiting factor, it makes total sense your HDD is a factor 10 slower.

Your internet connection isn't the bottleneck. There are only 2 things that can improve performance:
1. Get a SSD, preferrably a good brand (Samsung Evo for instance), not a budget disk. Check some reviews on Tom's Hardware to make sure you get a good one. But even a budget SSD will be much faster than your current HDD.
2. Add more RAM.

If you do both, you're entire computing experience will get much smoother.