Return a record with PL/pgSQL function - to speed up the query

Using OUT parameters achieve basically the same thing as in @klin's answer, but without creating user-defined types. Just move all your variables from the declare block into the argument-list as OUT parameters:

create or replace function get_user_info(
    IN  _id varchar,
    OUT is_banned boolean,
    OUT reputation integer,
    OUT is_vip boolean,
    OUT completed_games integer
)
-- no returns clause necessary, output structure controlled by OUT parameters
-- returns XXX
as $BODY$
begin
    select true into is_banned from pref_ban where id=_id;

    select
    count(nullif(nice, false)) -
    count(nullif(nice, true)) 
    into reputation
    from pref_rep where id=_id;

    select vip > now() into is_vip from pref_users where id=_id;

    select completed into completed_games from pref_match where id=_id;

    -- no return statement necessary, output values already stored in OUT parameters
    -- return XXX;
end
$BODY$ language plpgsql;

This will return a record (exactly one), so you can select its values as a normal record:

-- this will return all properties (columns) from your function:
select * from get_user_info();

-- these will return one property (column) from your function:
select is_banned from get_user_info();
select (get_user_info()).is_banned;

You should define a composite type. You can use it as return type of function and for record variables inside a function.

Example:

create type user_type as (
    is_banned boolean,
    reputation integer,
    is_vip boolean,
    completed_games integer);

create or replace function check_user_type ()
returns user_type language plpgsql as $$
declare
    rec user_type;
begin
    select true into rec.is_banned;
    select 100 into rec.reputation;
    select false into rec.is_vip;
    select 22 into rec.completed_games;
--  you can do the same in a little bit nicer way:
--  select true, 100, false, 22 into rec
    return rec;
end $$;

select * from check_user_type();

In my opinion using functions like this is quite reasonable in terms of both performance and application logic.

---

User-defined composite types are very useful if you want to return set of rows from your function. Then you should define return type of the function as setof composite-type and use return next or return query.

Example:

create or replace function check_set_of_user_type ()
returns setof user_type language plpgsql as $$
declare
    rec user_type;
begin
    for rec in
        select i/2*2 = i, i, i < 3, i+ 20
        from generate_series(1, 4) i
    loop
        return next rec;
    end loop;

    return query 
        select true, 100+ i, true, 100+ i
        from generate_series(1, 2) i;
end $$;

select * from check_set_of_user_type();

 is_banned | reputation | is_vip | completed_games
-----------+------------+--------+-----------------
 f         |          1 | t      |              21
 t         |          2 | t      |              22
 f         |          3 | f      |              23
 t         |          4 | f      |              24
 t         |        101 | t      |             101
 t         |        102 | t      |             102