Variation between query runtimes

Hi,

I have the following quering plans:

"Seq Scan on ind_uni_100 (cost=0.00..27242.00 rows=1000000 width=104)
(actual time=0.272..2444.667 rows=1000000 loops=1)"
"Total runtime: 4229.449 ms"

and

"Bitmap Heap Scan on ind_uni_100 (cost=314.00..18181.00 rows=50000
width=104) (actual time=74.106..585.368 rows=49758 loops=1)"
" Recheck Cond: (b = 1)"
" -> Bitmap Index Scan on index_b_ind_uni_100
(cost=0.00..314.00rows=50000 width=0) (actual time=
61.814..61.814 rows=49758 loops=1)"
" Index Cond: (b = 1)"
"Total runtime: 638.787 ms"

from pg_stast_get_blocks_fetched i can see that both queries need almost the
same number of disk fetches which is quite reasonable ( the index is
unclustered).

But as you can see there is a great variation between query
runtimes.Cansomeone explain this differnce?

Thanks!