postgresql performance

Dear Friends,
I have a table with 50 lakhs records, the table has more then 10
fields, i have primary key, i have select query with count(*) without any
condition, it takes 17 seconds.

I have another one query which will do joins with other small tables, it
takes 47 seconds to give output, the result has 2 lakhs records. the
indexing is not used. I have created one index with one field ( which i
used in this query, the field value has duplicates also ).

Now the cpu usage is 4.4%, and waiting for io process .

i have checked the above in the top command.

If i delete that index, then it works fine, now the cpu usage takes more
then 70%, and waiting shows less then 30%.

Can Anyone explain, Why postgres behaves like this.

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