That's not how hard drives work, and doesn't take into account that OP might want to download more than one thing at a time.
Hard drives are fastest when they are moving large single files. SSDs are way better than hard drives at lots of small random reads/writes.Setting qbittorrent up so that all the random writes inherent to downloading a torrent go to a small ssd, and then moving that file over to the big hard drive with a single long writer operation is how you make both devices perform to their best.