1. That's generally what they are supposed to do. Act like separate users, but depending on the content (if it's not unique to them) you need to start caching it. Either the whole page (Varnish / Nginx) or object based caching (Memcached / Redis).
2. They probably have a DDoS detection on their own APIs. So you need to start caching it, or buy a higher rate from them, so you can do that many queries.
3. Don't have any experience with cypress, so can't be of any assistans.
Based on your CPU and Memory load, you can without any problem have higher PHP-FPM and Nginx settings, depending on what dies first.
Your welcome, but these aren't really CodeIgniter problems, as this can applied to any application.
