nodebb-plugin-cdn
This plugin allows you to automatically redirect requests to a CDN. It has been tested with AWS Cloudfront, but other services will likely also work.
There are setup instructions for AWS Cloudfront here. If you want to get the absolute best performance, you'll want to use nginx to handle redirects as described here.
Make sure, prior to setup, that your nginx config does not have any location directives which apply to /plugins or /assets.