why does count take so long?

On a 7.3.4 database:

explain analyse select count(*) from elog;


  Aggregate  (cost=223764.05..223764.05 rows=1 width=0) (actual
time=81372.11..81372.11 rows=1 loops=1)
    ->  Seq Scan on elog  (cost=0.00..203012.24 rows=8300724 width=0)
(actual time=3.91..71542.53 rows=8313762 loops=1)
  Total runtime: 81378.42 msec
(3 rows)


It looks like the aggregate took 10 secs all by itself.  What's taking
so long?