Update Spends 1800 Times More Than Select

Hello,

The select statement needs only 1 second to complete the query.
But the update statement spends 30 minutes. Why?


SELECT STATEMENT:
declare @IDate smalldatetime
select @IDate=col001 from USDay
select * from USDay A
join (
select US990010, US990020, US990030, US990040, US990050, US990060,
US990070 from US99000D where US990010=@IDate
) B on A.col001=B.US990010 and A.col002=B.US990020
where B.US990010 is not null


UPDATE STATEMENT:
update US99000D
set US990030=A.col003,
US990040=A.col004,
US990050=A.col005,
US990060=A.col006,
US990070=A.col007
from USDay A
join (
select US990010, US990020, US990030, US990040, US990050, US990060,
US990070 from US99000D where US990010=@IDate
) B on A.col001=B.US990010 and A.col002=B.US990020
where B.US990010 is not null


INDEX:
clustered index: US990020, US990010
non-unique index: US990010, US990020