Count in PostgreSQL is slow because of MVCC.
SELECT COUNT(*) FROM table_name;
However, if you dont care the exact count, you can do fast estimate.
Replace table_name with real table name:
SELECT reltuples::BIGINT AS estimate
FROM pg_class
WHERE oid = 'schema_name.table_name'::regclass;
You can get schema_name by
SELECT nspname FROM pg_catalog.pg_namespace;