Slowing UPDATEs inside a transaction

Hi. I've only been using PostgreSQL properly for a week or so, so I
apologise if this has been covered numerous times, however Google is
producing nothing of use.

I'm trying to import a large amount of legacy data (billions of
denormalised rows) into a pg database with a completely different schema,
de-duplicating bits of it on-the-fly while maintaining a reference count.
The procedures to do this have proven easy to write, however the speed is
not pretty. I've spent some time breaking down the cause and it's come down
to a simple UPDATE as evidenced below:

CREATE TABLE foo (a int PRIMARY KEY, b int);
INSERT INTO foo VALUES (1,1);

CREATE OR REPLACE FUNCTION test() RETURNS int AS $$
DECLARE
i int;
BEGIN
FOR i IN 1..10000 LOOP
UPDATE foo SET b=b+1 WHERE a=1;
END LOOP;
RETURN 1;
END;
$$ LANGUAGE plpgsql;

When run individually, this function produces the following timing:
Time: 1912.593 ms
Time: 1937.183 ms
Time: 1941.607 ms
Time: 1943.673 ms
Time: 1944.738 ms

However, when run inside a transaction (to simulate a function doing the
same work) I get this:

START TRANSACTION
Time: 0.836 ms
Time: 1908.931 ms
Time: 5370.110 ms
Time: 8809.378 ms
Time: 12274.294 ms
Time: 15698.745 ms
Time: 19218.389 ms

There is no disk i/o and the postgresql process runs 100% cpu.
Server is amd64 FreeBSD 8-STABLE w/16GB RAM running postgresql 9.0.3 from
packages

Looking at the timing of real data (heavily grouped), it seems the speed of
UPDATEs can vary dependent on how heavily updated a row is, so I set out to
produce a test case:

CREATE TABLE foo (a int PRIMARY KEY, b int);
INSERT INTO foo VALUES (1,1),(2,1),(3,1),(4,1);

CREATE OR REPLACE FUNCTION test(int) RETURNS int AS $$
DECLARE
i int;
BEGIN
FOR i IN 1..10000 LOOP
UPDATE foo SET b=1 WHERE a=$1;
END LOOP;
RETURN 1;
END;
$$ LANGUAGE plpgsql;
START TRANSACTION;
SELECT test(1); Time: 1917.305 ms
SELECT test(2); Time: 1926.758 ms
SELECT test(3); Time: 1926.498 ms
SELECT test(1); Time: 5376.691 ms
SELECT test(2); Time: 5408.231 ms
SELECT test(3); Time: 5403.654 ms
SELECT test(1); Time: 8842.425 ms
SELECT test(4); Time: 1925.954 ms
COMMIT; START TRANSACTION;
SELECT test(1); Time: 1911.153 ms

As you can see, the more an individual row is updated /within a
transaction/, the slower it becomes for some reason.

Unfortunately in my real-world case, I need to do many billions of these
UPDATEs. Is there any way I can get around this without pulling my huge
source table out of the database and feeding everything in line-at-a-time
from outside the database?

Thanks.

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