Honestly I think this is because Notion, Slite are mostly frontend apps at the core. And thus attract talents who are strong in frontend, and the culture is bia

Honestly I think this is because Notion, Slite are mostly frontend apps at the core. And thus attract talents who are strong in frontend, and the culture is biased toward front end (actually in Notion it is even biased toward UI). And thus they neglect backend due to their culture which leads to performance issues. I agree that you can't do Notion in a weekend, I have been working on something similar and it took me 8 months full time to have something that starts to look ok. But that's also why I'm not bearish on having good performance in that kind of app. Notion made very dubious choices regarding their data model which explains why it is slow. Slite did not, but I thought that Slite had better performance though