It gets a lot faster if you run the function in a subquery and only decompose the resulting row type in the outer SELECT list:

SELECT s.schemaname, s.relname, st.*
FROM   pg_statio_user_tables s
CROSS  JOIN LATERAL pgstattuple(s.relid) AS st
ORDER  BY 1, 2;  -- optional

This way, the function is only called once per row in pg_statio_user_tables. You were stumbling over a weak spot in the Postgres query planner. See:

• How to avoid multiple function evals with the (func()).* syntax in a query?

Since Postgres 9.4 pgstattuple() can take an OID, which is preferable.

CREATE EXTENSION — loads a new extension into the current database.

So, you could do something like:

CREATE EXTENSION pgstattuple;