I am running into speed-issues for big files, whose extension is recognized by Notepad++ which will be syntax-highlighted.
Is there a Shortcut or option to assign keys to toggle Syntax-Highlighting in Notepad++?
The only work around I know of is to click on Language from the context menu and select the last option
User-Defined (assuming it's not in use). This will remove all highlighting. Then, to put it back on, you can select the language again. All of this is with the mouse though.
To use the keyboard, the 'shortcut' would be alt + L + U (so, not really a shortcut). Then, to put the language back you'd have to do the same but for your language. For example alt + L + H + HEnter would reset it to HTML
You can create a macro with the commands presented by @daverook and assign a keyboard hotkey to it and enable/disable that way too. Let me know if you need help with that.
UPDATE: I filed a request with the developer to make this an included feature in the application and hopefully will be implemented in next version.