Getting row with id=max(id)

I'd like to retrieve a row of a table that has the maximum ID. For example,
with:

id | s
----+-------
1 | alpha
2 | beta
3 | gamma
4 | delta

I'd like to get the row with ID=4. I've tried:

SELECT * FROM mytable WHERE id=(SELECT MAX(id) FROM mytable);

The subquery can take a /really/ long time on a table that is large. The query:

SELECT * FROM mytable ORDER BY id DESC LIMIT 1;

doesn't seem to help very much. What query is the fastest at getting this row?

A related question is: is there a way to time a query in psql, like the
client of MySQL does?