I have 18.413 notes in my Zettelkasten with 3.2 million words in total.
Performance is still good, although I do suffer from high first search time (2+ minutes, reported here) and layout shifts when navigating, which makes the experience less pleasant and makes me lose position in documents easily.
Startup times are still good (less than 15 seconds I guess). Creating a new note still goes quickly. Navigating is still quick for me. I don’t experience a high memory usage (463mb at the moment, which is pretty typical for Electron apps).
Obsidian rarely if ever crashes. If my memory is correct, Obsidian closed/crashed 2 times on me this year (with daily use), and in neither case I lost work.
I don’t use the graph plugin anymore; I have 57.800 internal links and it takes around 4 minutes before the entire graph is build, which is not worth it for me.