flyspell-large-region

flyspell-large-region is a variable defined in `flyspell.el'.
Its value is

1000

Documentation:
The threshold that determines if a region is small.
If the region is smaller than this number of characters,
`flyspell-region' checks the words sequentially using regular
flyspell methods. Else, if the region is large, a new Ispell process is
spawned for speed.

Doubled words are not detected in a large region, because Ispell
does not check for them.

If this variable is nil, all regions are treated as small.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.