This applies to machines having little RAM resources (e.g 512Mb).
When a machine swaps, data is actually written on the disk rather than the real RAM. This is considerably slower. Swap tendency can be put off completely (value 0) or put at maximum (value 100). The latter will make the machine constantly write to the swap.
To check the current value of swappiness in your system, do:
The default value in Ubuntu based distributions is 60. This value is fine for servers but considered high for home users. You may lower it by adding the following to the /etc/sysctl.conf file.
# Reduce the swap tendency vm.swappiness = 10