With everything as a note, how was it so performant? How did it scale so well?

It was basically NoSQL before NoSQL.

Each note was just a record, but with no schema. Schemas were imposed at the UI layer by forms and at indexing time by views.

Baffling to see this, in every place I've worked at that used Lotus Notes, it was an absolute dog on the system. Clunky, slow, and ground everything else to a halt. And this was the case even on a relatively modern laptop in 2019. Not what I'd call performant at all!