the best performance looks to be dedicating each task to its own hdd, that way no one task bottlenecks any other thats also running, which is what is creating the performance drop youre seeing.
so, what you should do then is seperate your hdds. put your os and apps on one hdd keeping its performance seperate (ideally one of your faster hdds, cuz its sucks having a slow os), and have 3 seperate dedicated active torrenting hdds, each having their own active set of incomplete uploads and downloads. and another seperate hdd for where completed downloads automatically transfer to when theyre finished and for other general storage, you can also use your seperate storage hdd for even more simultaneous parallel torrenting, if you wanted to. using your storage hdd for uploading and downloading torrents isnt recommended though, because performance might suffer with completed transfers also being sent there simultaneously from the 3 other torrenting hdds. your storage hdd isnt a performance hdd anyhow, its just there for storage purposes.
another option is to have an os and app hdd, 2 parallel torrenting hdds, and then 2 hdds for simultaneous completing torrent transfers and other storage purposes (raid 0 is not recommended for storing important data, unless the data is backed up elsewhere too for redundancy, you should always have a backup of your important data anyhow, because mechanical hdds do fail, and will fail, its just a matter of when). either way, parallel torrenting seems to be the answer, moreso than raid.
another option for either is to just keep the completed torrent directories on their respective torrenting hdds, to minimize unnecessary transferring and bottlenecks between hdds. this actually seems the most ideal setup from a performance and capacity perspective.
the reason for raid 0 not being ideal for this is when hdds are in raid 0, the heads all move as one, keeping optimal performance limited to only one transfer at at time, which is not good for multitasking.
also, im not sure defragging torrents is entirely necessary either, even if they are heavily fragmented, which should allow the individual hdds to keep performing well too.
as far as torrenting in parallel, you might need to launch a seperate torrenting application/process for each hdd