Indicates a fast terminal connection. More characters will be sent to the screen for redrawing, instead of using insert/delete line commands. Improves smoothness of redrawing when there are multiple windows and the terminal does not support a scrolling region. Also enables the extra writing of characters at the end of each screen line for lines that wrap. This helps when using copy/paste with the mouse in an xterm and other terminals.

The default used to be set only for some terminal names, but these days nearly all terminals are fast, therefore the default is now "on". If you have a slow connection you may want to set this option off, e.g. depending on the host name:

if hostname() =~ 'faraway'
   set nottyfast
endif

(default on)