While I'm not sure what specifically might cause a dramatic loss of frames in NW.js (in terms of things you do with your game event-wise), it's no surprise that games generally run worse with that export option.
A little thing you could try is use a current version of NW.js instead of only the C2 export. In order to do so you can simply copy the file 'package.nw' from your export to the extracted folder of the downloaded new NW.js version and it will run.
In my limited experience it runs noticably better, but sadly not as flawless as it might appear directly in the browser.