VIM + Syntastic: how to disable the checker?

I have disabled Syntastic by default and activate/disable error checking with the following in my .vimrc:

let g:syntastic_mode_map = { 'mode': 'passive', 'active_filetypes': [],'passive_filetypes': [] }
nnoremap <C-w>E :SyntasticCheck<CR>

When I need to use error checking I simply hit: ctrl-w E

Using :SyntasticToggleMode you can toggle Syntastic into passive mode, which will disable auto-checking. You can then check a file by running :SyntasticCheck instead.

For more, see :help syntastic-commands

On another note: if Syntastic is slow for you consider trying ale as an alternative. Unlike Syntastic it runs asynchronously, so even if it's slow it shouldn't hinder you.