Transfer speeds are hindered by three factors. Bandwidth, Protocol, and Disk speeds.
This really doesn't seem to to be the issue - as you've mentioned you're on a LAN - ideally there isn't much to go wrong on a LAN except for other users taking more bandwidth on the network or poor reception (be it WIFI or long distance cable).
This is where more overhead comes into play. You compared speeds of Bittorrent and SFTP - well those protocols differ extremely. Bittorrent is a highly compressed, multi-threaded, multi-seed (multi-peer) file transfer protocol. SFTP is FTP over SSH which can add some overhead especially when paired with and Bandwidth issues.
Hard drives are the last thing people suspect with talking about download or upload speeds. However, you can only save (and write) data as fast as your hard drives can spin (unless you're using an SSD (Solid State Disk) in which case this does not pertain to you).
I noticed you mentioned that you can download from Bittorrent to your laptop and experience poor uplink speeds to your server. Have you tried to run speed tests from Internet to your home server? That should help you benchmark speeds from "Internet" to server. Secondly, ensure you don't have any other major traffic on your network when you're running these tests (like having Bittorrent open) as it'll produce invalid results.