How feasible is a Drupal 8 with a different language depending on its subdomain So, let's say I have this massive content-heavy site with all sorts of paths like mysite.org/category/date/slug or mysite.org/someterm/subterm/date/slug. Let's say all this content is in English, and now I've gotten my hands on a Spanish version of all of my content. How feasible is it to integrate the Spanish content into the site, using the same installation, but only having the site show up in Spanish when people go to es.mysite.org, es.mysite.org/category, or es.mysite.org/someterm/subterm/date/slug. 

How scalable is Drupal 8 in this regard? If we want to have several different language sites, potentially with different templates, would it be best to use different installations of Drupal 8 or does it facilitate this kind of set up I explained above so that one installation is perfectly logical?
Multilingual has seen a big improvement for Drupal 8. All you need can be done with Drupal 7, so it it's possible with Drupal 8 too.

You have to set Language Negotiation to subdomain. This way Drupal sets the content language depending on the domain accessed. For example, example.com/en/node/1 is English content and example.com/es/node/1 is Spanish content (you can also configure so no subdomains maps to a certain language).

Regarding performance Drupal 8 has a very good caching system that is much better than Drupal 7 (specially when we talk about cache invalidation).

What I don't recommend in any case is to have two separate Drupal installations one for each language. No benefits from this approach and lot of issues instead. Just think on content sync, user sync, config sync, etc. Drupal can manage two and much more languages without having performance issues.