Em :h autoload
:
AUTOMATICALLY LOADING FUNCTIONS
autoload-functions
When using many or large functions, it's possible to automatically define them
only when they are used. There are two methods: with an autocommand and with
the "autoload" directory in 'runtimepath'.
...
Using an autoload script
autoload E746
This is introduced in the user manual, section 41.15.
Using a script in the "autoload" directory is simpler, but requires using
exactly the right file name. A function that can be autoloaded has a name
like this:
:call filename#funcname()
When such a function is called, and it is not defined yet, Vim will search the
"autoload" directories in 'runtimepath' for a script file called
"filename.vim". For example "~/.vim/autoload/filename.vim". That file should
then define the function like this:
function filename#funcname()
echo "Done!"
endfunction
The file name and the name used before the # in the function must match
exactly, and the defined function must have the name exactly as it will be
called.
Portanto, autoload
é um diretório no qual você pode eliminar arquivos que definem funções, e o arquivo só é lido quando a função é chamada pela primeira vez. Isso torna a inicialização mais rápida.