If you have downloaded applications thru torrents, and their made your computer slow, I would rather recommend you re-format and re-install your computer from scratch. There's good chance it has been compromised with viruses and/or malware.
your slow downloading could be you have torrents downloading/uploading in the background and not know it. whatever you use for torrents make sure it is closed. but with torrents it can be viruses too so i agree with the above