Whether it's using Deluge to download torrents, or downloading files from a browser, or
mving files, on main Ubuntu 16.04 SSD, when this happens the system tends to lag / freeze / hang for either a long period of time, or short bursts until either the file is entirely written, or entirely downloaded.
vm.dirty_background_ratio = 5 vm.dirty_ratio = 10 vm.swappiness = 0
cat /proc/sys/vm/dirty_background_bytes 0 cat /proc/sys/vm/dirty_bytes 0
Some more information is that it's a newer SSD, with 16GB of RAM and at least 8GB of swap, and there is at least 30GB left on the SSD.
Any sort of copying, writing, downloading, torrenting large files (usually over 600MB or if there are multiple files) seems to lead to long lags or short bursts depending on how slow or fast the actual write is.