I had over 1300 notes in Apple Notes and performance was still great: fast search, sync, note entry etc.. Maybe it slows down with more notes, but I think we need more real life examples to draw conclusions. In the past Apple Notes had a bug which made it slow down even with only a few notes.
I am now keeping the same amount of notes in IA Writer and 1Writer and both apps perform great.
Yeah, I have been using iA Writer as my primary recently. I really really like it. Simplistic, easy, … fast.