10,000+ nodes vs. 1 node with 10,000+ field collection items I need a sort of term database with around 3 fields that won't be used during normal site operations.. Only when I, the administrator, add content.

I'm wondering about how to minimise performance impact for regular users.. My intuition tells me that a field collection would be better so the nodes table is smaller.

I will be likely be using some code to add data from APIs and Rules to fetch the data by property.

Which should i use? I don't have much interest in creating a custom entity type unless it's really easy.. I want image and audio fields and I don't know if that complicates things.
Loading 1 node with like 10,000+ field collection items sounds like performance suicide if you don't limit the amount of items being loaded and displayed initially. And having to open a node with 10,000+ field collection items only to maybe add or edit one item will be really big pain then.

With nodes, much likely you'll never display all of them at once. You very likely will have them organised in Views. Limited display, pagers, filters, lots of caching possibilities. When you want to add a node, you'll have a fresh node form, loaded in milliseconds. Plus you will have a much larger number of contrib modules available dealing with sortability, searchability, orderability and a lot of other -abilities that come in handy when organising content. Also when it comes to content import.

Go for 10,000+ or even 1,000,000+ nodes. That's what Drupal is made for.