We've been extremely happy with the Python 3 (with mypy static types) and Postgres stack. Like with any app, you need to take some care to design the database model right and avoid database queries in loops. But with our design, all the expensive operations are done in the database and cache, so Python is just as fast as anything else. And because our data model design and indexes are correct and Postgres is awesome, the database queries are all fast. We use Postgres' built-in full-text search, and people frequently praise Zulip's search as being unusually good for finding past conversations.

For Zulip's scalability, see https://zulip.readthedocs.io/en/latest/production/maintain-s...