When copying large amounts of data, e.g. digital images to an usb drive, I noticed laggy system behaviour, to the point that the mouse would not move.
The problem seems to be cache queue bloat, which I could reduce by adding these settings to my local sysctl config file (/etc/sysctl.d/my-sysctl.conf>:
vm.swappiness=10 vm.dirty_background_ratio = 5 vm.dirty_ratio = 10
I had to reboot the system because sudo sysctl -p did not reload the values. For more details on this see the references
Ref: