Insert text is very slow in Vim

Read from the local file (super fast):

:read file

Try to set syntax off:

:set syntax=off

Vim tries to keep your work safe and doesn't assume you can type several thousand characters per second. Read :help swap-file for some details on the buffering. The solution to your problem is this:

Turn off vim's swapfile while you are pasting text:

:set noswapfile

Turning off swap is not safe for normal operations! Immediately after the paste:

:set swapfile

See :help swapfile for more details.

Folding could be the problem:

set foldenable              " can slow Vim down with some plugins
set foldlevelstart=99       " can slow Vim down with some plugins
set foldmethod=syntax       " can slow Vim down with some plugins

According to vim's help (:help foldmethod), the syntax setting caused vim to use syntax highlighting to determine how to automatically fold my code.

Other things to check/toggle are syntax, filetype, wrap and line length (some plugins can be slow with very long lines).

Running Vim without your current settings is a good starting point:

# vim -u NONE

Reference:

http://stackoverflow.com/questions/15086155/vim-insert-mode-is-very-slow-with-400-lines
http://superuser.com/questions/550669/my-copy-of-vim-is-running-extremely-slowly-when-i-edit-medium-to-large-eg-1000