For a faster response use https://stallion-theme.co.uk/wordpress-seo-comments-plugin/ for support.
The free plugin will work fine with large sites, I’ve had it running for years on a site with a few thousand posts and over tens of thousands of comments. Some of the posts have thousands of comments each.
The way the plugin works is nothing new is added to the server, uses the database entries (the comments) that’s already on your server, so nothing new is created, so no additional server load per se.
That being said, Google etc… will index all the SEO comments, so with thousands of comments Google will index thousands of extra webpages. And if the comments rank for SERPs, more traffic from users as well.
So there will be more search engine bots spidering your site. Because the URLs to the comments are dynamic URLs Google automatically spiders them slower than other webpages: this is a Google feature for if a site is mis-configured and the same content can be indexed with dozens of different dynamic URLs. As long as you have an OK hosting plan, should be OK, been using this for years and I’ve not had an issue and I use cheap Godaddy VPS servers.
On excluding posts, with the free plugin there’s no options to set character limits etc… I have added that option to the Stallion Responsive theme version of the plugin (built in theme feature, see: https://stallion-theme.co.uk/stallion-seo-super-comments-plugin/).
With the Stallion Responsive Theme version there’s character limits and other built in plugins that work alongside the SEO super comments, for example a comment title feature, so you and your users can set a title to comments and the titles are used as the title tags and anchor text on the SEO comments adding additional SEO benefits. Example Stallion SEO Super Comment output: https://stallion-theme.co.uk/stallion-seo-super-comments-plugin/?cid=17766
On the site I’ve linked to above currently have the character limit set to 1,000 so only large comments are linked to. Look through a few of the comments and you’ll see what’s possible with Stallion Responsive.
I’ve not tried to add a word filter, that would be problematic so not on the list of future features.
David