Slow query + why bitmap index scan??

This will be simple question to answer. :-) There is a single table:

select count(*) from product_price_history -- 12982555 rows

This table has exactly one index and on primary key constraint:

CREATE INDEX idx_product_price_history_id_hdate
ON product_price_history
USING btree
(id, hdate);

ALTER TABLE product_price_history
ADD CONSTRAINT pk_product_price_history PRIMARY KEY(hid);

No more constraints or indexes defined on this table. Rows are never
updated or deleted in this table, they are only inserted. It was
vacuum-ed and reindex-ed today.

Stats on the table:

seq scans=13, index scans=108, table size=3770MB, toast table size=8192
bytes, indexes size=666MB

This query:

select hid from product_price_history where id=35547581

Returns 759 rows in 8837 msec! How can this be that slow???

The query plan is:

"Bitmap Heap Scan on product_price_history (cost=13.90..1863.51
rows=472 width=8)"
" Recheck Cond: (id = 35547581)"
" -> Bitmap Index Scan on idx_product_price_history_id_hdate
(cost=0.00..13.78 rows=472 width=0)"
" Index Cond: (id = 35547581)"

I don't understand why PostgreSQL uses bitmap heap scan + bitmap index
scan? Why not just use an regular index scan? Data in a btree index is
already sorted. A normal index scan should take no more than a few page
reads. This sould never take 8 seconds.

Thanks,

Laszlo