Summary:
Every downloaded file is heavily fragmented. For example a 5.1GB file at 59.6% completion has 2333 fragments reported by Piriform Defraggler.
Version:
Deluge Client 1.3.13
libtorrent 1.0.9.0
Windows 10 Pro 64bit, x64-based processor
Hard disks with plenty of free space, formatted NTFS
Plugins enabled:
Label
Scheduler
Language: English
Installation method: deluge-1.3.13-win32-py2.7-0.exe installer
Allocation: set to Use Full Allocation
Popup states this setting preallocates all of the space that is needed for the torrent and prevents disk fragmentation.