Index scan startup time

[Apologies if this already went through. I don't see it in the archives.]

Normally one expects that an index scan would have a startup time of nearly
zero. Can anyone explain this:

EXPLAIN ANALYZE select activity_id from activity where state in (10000, 10001)
order by activity_id limit 100;

QUERY PLAN

Limit (cost=0.00..622.72 rows=100 width=8) (actual
time=207356.054..207356.876 rows=100 loops=1)
-> Index Scan using activity_pk on activity (cost=0.00..40717259.91
rows=6538650 width=8) (actual time=207356.050..207356.722 rows=100 loops=1)
Filter: ((state = 10000) OR (state = 10001))
Total runtime: 207357.000 ms

The table has seen VACUUM FULL and REINDEX before this.

The plan choice and the statistics look right, but why does it take 3 minutes
before doing anything? Or is the measurement of the actual start time
inaccurate? This is quite reproducible, so it's not just a case of a
temporary I/O bottleneck, say.

(PostgreSQL 8.0.3)

--
Peter Eisentraut
http://developer.postgresql.org/~petere/