SQL Server :: Query Slow In Loading / Trying To Display Query Result?

I am trying to display this query result in an aspx page.

It is very slow in loading. Here is the query. The inner query inside the outer quesry is the problem.(Please see the underlined part in the query) - (If I remove that part it is very fast.)

select

top 500

--This column is the issue
,Governing_Class=( case when exists (select top 1 tqc.class_code from
t_quote_class tqc
inner join t_quote_class_premium tqcm on tqc.class_code =tqcm.
class_code
where tqc.appid=pi.appid and tqc.class_code not in('8742' ,'8810','7380')
order by tqcm.premium_amt desc
)
then ( select top 1 tqc.class_code from
t_quote_class tqc
inner join t_quote_class_premium tqcm on tqc.class_code =tqcm.
class_code
where tqc.appid=pi.
appid
order by tqcm.premium_amt desc
)
......... From tables