The list endpoint is slow but every query is fast

• GET /users takes 900 ms to return 100 users.
• Every query in the slow-query log is sub-millisecond.
• The database log shows the same SELECT repeated 100 times with different ids.

-- database log, in one request:
SELECT id, name FROM users LIMIT 100;
SELECT count(*) FROM orders WHERE user_id = 1;
SELECT count(*) FROM orders WHERE user_id = 2;
SELECT count(*) FROM orders WHERE user_id = 3;
... (97 more)