Related, people not understanding that the latency alone from a DB call (especially when the DB has network-based storage, which is most of the cloud offerings) is substantial. I’ve found so many cases where people are unknowingly doing serialized queries because the ORM obscured everything, and they didn’t realize they could get everything in a single query with joins.
Yeah, I have that problem right now. A colleague and I recently took over development of an application that two others have started. It's not even in prod yet but everything is slow. Loading a page can take up to a minute.
The reason for this is they have a frontend where they do stuff like
let a = await fetchA();
let b = await fetchB(a.id);
let c = await fetchC(b.id);
// ...
This on its own is obviously slow. But then you look at the backend code and it turns out the backend code for A, B and C also do this same kind of thing. Even worse, it's fetching way more data than is actually needed so even a single request is quite slow simply because there's a lot of data. And worse yet, it's fetching the data in a shape that is completely wrong for the application, so the frontend has thousands of lines of code that reshapes the data before it can be used - making things even slower, and making our jobs difficult because it's really hard to wrap your head around everything that's happening.
And there's no technical reason for it, it doesn't have to be like this. We're working on refactoring it at the moment, we have a deadline approaching quite fast so we don't really want to spend time refactoring but the system they've created is complete ass and we eventually decided we have to spend the time to refactor now. It'll be much more difficult to fix the database schema and such when we're in prod and need to worry about losing data.