The default Ubuntu settings do not allow Filezilla to download at full speed (each thread is limited to 800kb/s by default).
To erase this limitation, type in terminal:
sudo sysctl -w net.ipv4.tcp_rmem=‘40960 873800 62914560’
sudo sysctl -w net.core.rmem_max=8000000
Unfortunately you must type this every time you restart the computer.
Source: https://forum.filezilla-project.org/viewtopic.php?t=40513