I'm using Syntastic which is enabled for my HTML files. Since I have a very big file with "validator w3" checkers enabled, GVIM or VIM became very slow while saving the file (:w).
Is it possible to toggle syntastic off temporally just for the current session?
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.