I work at Notion.
It’s stock RDS on EBS, which as you say is super slow compared to Genuine Local Storage. Any large scan or large result set query that doesn’t fall into the Postgres cache or the underlying EBS cache is frustratingly slow.
However almost all of our reads are “get row by ID” lookups that hit memcached. Our query pattern is mostly recursive graph traversal which Postgres is bad at, with our table size even a 20x faster disk wouldn’t make it feasible to do in SQL. We also copy paste “notion database” data in to a stateful caching service, but that also needs specialized indexing for user defined schema, another thing Postgres can’t handle (my last project was an improvement to that service).
The place we really suffer from the EBS slowness is with full table scan data migrations, stuff like “for all blocks in Notion, block = f(block)”. For this we use “Big Data” approach reading from DB dumps in S3 instead of Postgres directly.
Ultimately I’d prefer to serve every read from your device’s local storage, which is my current project.
