Delete cascade produce scan intead of index seek
You may want to look in the table sys.foreign_keys for the foreign key joining the tables, and see if the index has the value is_not_trusted = 1.

This query, originally from BrentOzar.com, will show you which foreign keys in your system are not trusted:

SELECT '[' + s.name + '].[' + o.name + '].[' + i.name + ']' AS keyname
from sys.foreign_keys i
INNER JOIN sys.objects o ON i.parent_object_id = o.object_id
INNER JOIN sys.schemas s ON o.schema_id = s.schema_id
WHERE i.is_not_trusted = 1 AND i.is_not_for_replication = 0;
GO
SELECT '[' + s.name + '].[' + o.name + '].[' + i.name + ']' AS keyname
from sys.check_constraints i
INNER JOIN sys.objects o ON i.parent_object_id = o.object_id
INNER JOIN sys.schemas s ON o.schema_id = s.schema_id
WHERE i.is_not_trusted = 1 AND i.is_not_for_replication = 0 AND i.is_disabled = 0;
GO


Note: I recommend at least subscribing to Brent Ozar's emails - lots of interesting information coming out of that website, and their Blitz queries are really helpful.