postgresql is slow with larger table even it is in RAM

I have a table with 32 lakh record in it. Table size is nearly 700 MB,
and my machine had a 1 GB + 256 MB RAM, i had created the table space in
RAM, and then created this table in this RAM.

So now everything is in RAM, if i do a count(*) on this table it returns
327600 in 3 seconds, why it is taking 3 seconds ????? because am sure that
no Disk I/O is happening. ( using vmstat i had confirmed, no disk I/O is
happening, swap is also not used )

Any Idea on this ???

I searched a lot in newsgroups ... can't find relevant things.... ( because
everywhere they are speaking about disk access speed, here i don't want to
worry about disk access )

If required i will give more information on this.