SQL - Make this statement faster i.e. less duration, reads - linq-to-sql

Question

Given the following tables:
Orders (OrderID, OrderStatus, OrderNumber)
OrderItems(OrderItemID, OrderID, ItemID, OrderItemStatus)
Orders: 2537 records
Order Items: 1319 records
I have created indexes on
Orders(OrderStatus)
OrderItems(OrderID)
OrderItems(OrderItemStatus)
I have the following SQL statement (generated by LinqToSql) which when executed, has:
- duration = 8789
- reads = 7809.
exec sp_executesql N'SELECT COUNT(*) AS [value]
FROM [dbo].[Orders] AS [t0]
WHERE ([t0].[OrderStatus] = #p0) OR (EXISTS(
SELECT NULL AS [EMPTY]
FROM [dbo].[OrderItems] AS [t1]
WHERE ([t1].[OrderID] = [t0].[OrderID]) AND ([t1].[OrderItemStatus] = #p1)
))',N'#p0 nvarchar(2),#p1 nvarchar(2)',#p0=N'KE',#p1=N'KE'
Is there anything else which I can do to make it faster?

Answer 1

make all those nvarchars parameters varchars if the columns in the table are varchars
))',N'#p0 varchar(2),#p1 varchar(2)',#p0=N'KE',#p1=N'KE'
See also here: sp_executesql causing my query to be very slow

Answer 2

Count on a single index rather than *

Answer 3

This might generate some better sql.
IQueryable<int> query1 =
from oi in db.OrderItems
where oi.OrderItemStatus == theItemStatus
select oi.OrderID;
IQueryable<int> query2 =
from o in db.Orders
where o.OrderStatus == theOrderStatus
select o.OrderID;
IQueryable<int> query3 = query1.Concat(query2).Distinct();
int result = query3.Count();