Do standard records like Invoices and Sales Order have limitations on the number of lines they accept? The items? And what about custom records with child sublist?

Transactions are unusable in the UI after 500 or so lines

No it is just about performance, right? But there are no limits.

Even without worrying about the UI, you're going to have serverside script perf problems with more than a few thousand per transaction. I would not go above 2k or so on any transaction (actually have processes that split journals up). You'd need to seriously consider what you're doing & why if you're asking if there's an actual limit

One of our functional consultants wanted to be sure, and there are different answers on the internet. What I understand now is, the limit is actually 10K, but going over 500 lines is bad for the user experience.