Well, you aren't just doing a count, you are also asking for distinct rows (theres a sort operation going on). Therefore, you might expect it to take a little longer:

Code:
select count(i_id) from x;

COUNT(I_ID)
-----------
    1020399

Elapsed: 00:00:01.19

select count(distinct(i_id)) from x;

COUNT(DISTINCT(I_ID))
---------------------
               720449

Elapsed: 00:00:04.61