Their new CDN is for paid plans yes, so you should expect good performances for your hosted sites, and the number of the sites you have doesn’t count at all or affect the performances. What affect the performances of the CDN is the total of activity on a server at a time. And one server must be containing a very lot of webflows sites, not only yours. More than the number, it’s the activity, visits, hits they generate.
But the technology of today request more power on the users’ end than on the server. The server gives the html+js+css code along with the js lib files, it’s only transfer to the user, then the users’ computer and browser process the whole to build the pages and interactions. The old school technology, the one behind Wordpress for example, requires a PHP program on the server and a database. Any time a user request a page, the server runs PHP and picks data in the base to build a html page that is sent to the user. That’s a lot more computer power required and possibilities to end up with a weak or undersized hosting. These technologies have been built when end users’ computers were very weak. Now your iPhone or Android is kinda tremendously powerful so it allows for heavy use of Javascript and CSS anims.