Query rewrite for performance

Question

The query below runs for more than three hours to complete. This both the tables have more than 100 millions rows. Please suggest how we can rewrite or tune it.

;With CTE_TO
as
(
Select top 10000000 max(ID) as ID, FormID
FROM [DB_history].[dbo.[Form] with (nolock)
Where Code =1 group by FormID
)
,DelMax
as
(
SELECT a.FormID ,b.ODSID as Maxdeleterecord, max(a.ID) as ID
From [DB_History].[dbo].[Form] as a
inner join CTE as b on a.FormID=b.FormID
Group by b.ODSID,a.entrantFormID HAVING max(a.ID) = b.ID
)

delete b
from [DB.[dbo].[Form] b
where exists (Select 1 from DelMax a where a.FormID=b.FormID)

Answer 1

Check the execution plan to see if e.g. a suitable index is used or not = missing index.
You can share the execution plan by https://www.brentozar.com/pastetheplan/