Update takes longer than expected

Hi,

I have an update that takes longer than expected and wondered if
there's an easy way to make it go faster.

It's pretty simple:-

create table session (
  id serial primary key,
  data text);

update session
  set data = 'ipsum lorem...'
  where id = 5;

The "ipsum lorem.." stuff is an encrypted session variable from a
rails app that does tend to get quite large

select avg(length(data)) from session
=> 31275

We're trying to migrate the app from mysql to pg and this is one of
the performance bottle-necks.  Unfortunately it slows down every
request by about 5 seconds.  MySQL (both MyISAM and InnoDB) does this
almost instantaneously.

Cheers,
Andy