Query too slow - Optimization

I am having an issue with the following query returning results a bit too slow and I suspect I am missing something basic. My initial guess is the 'CASE' statement is taking too long to process its result on the underlying data. But it could be something in the derived tables as well.

The question is, how can I speed this up? Are there any glaring errors in the way I am pulling the data? Am I running into a sorting or looping issues somewhere? The query runs for about 40 seconds, which seems quite long. C# is my primary expertise, SQL is a work in progress. Any tips would be greatly appreciated!

  SELECT 
hdr.taker
, hdr.order_no
, hdr.po_no as display_po
, cust.customer_name
, hdr.customer_id
, 'INCORRECT-LARGE ORDER' + CASE 
                    WHEN (ext_price_calc >= 600.01 and ext_price_calc <= 800) and fee_price.unit_price <>  round(ext_price_calc * -.01,2)
                        THEN '-1%: $' + cast(cast(ext_price_calc * -.01 as decimal(18,2)) as varchar(255))
                    WHEN ext_price_calc >= 800.01 and ext_price_calc <= 1000 and fee_price.unit_price <>  round(ext_price_calc * -.02,2)
                        THEN '-2%: $' + cast(cast(ext_price_calc * -.02 as decimal(18,2)) as varchar(255))
                    WHEN ext_price_calc > 1000 and fee_price.unit_price <>  round(ext_price_calc * -.03,2)
                        THEN '-3%: $' + cast(cast(ext_price_calc * -.03 as decimal(18,2)) as varchar(255))
                    ELSE
                        'OK'
                  END AS Status
FROM
(myDb_view_oe_hdr hdr
LEFT OUTER JOIN myDb_view_customer cust 
ON hdr.customer_id = cust.customer_id)
LEFT OUTER JOIN wpd_view_sales_territory_by_customer territory
ON cust.customer_id = territory.customer_id
LEFT OUTER JOIN 
    (select
        order_no,
        SUM(ext_price_calc) as ext_price_calc
    from 
    (select
        hdr.order_no,
        line.item_id,
        (line.qty_ordered - isnull(qty_canceled,0)) * unit_price as ext_price_calc 
    from myDb_view_oe_hdr hdr
    left outer join myDb_view_oe_line line
    on hdr.order_no = line.order_no
    where 
        line.delete_flag = 'N'
        AND line.cancel_flag = 'N'
        AND hdr.projected_order = 'N'
        AND hdr.delete_flag = 'N'
        AND hdr.cancel_flag = 'N'
        AND line.item_id not in ('LARGE-ORDER-1%','LARGE-ORDER-2%', 'LARGE-ORDER-3%', 'FUEL','NET-FUEL', 'CONVENIENCE-FEE')) as line
    group by order_no)  as order_total
    on hdr.order_no = order_total.order_no
 LEFT OUTER JOIN 
    (select 
        order_no,
        count(order_no) as convenience_count
    from oe_line with (nolock)
    left outer join inv_mast inv with (nolock)
    on oe_line.inv_mast_uid = inv.inv_mast_uid
    where inv.item_id in ('LARGE-ORDER-1%','LARGE-ORDER-2%', 'LARGE-ORDER-3%')
        and oe_line.delete_flag <> 'Y'
    group by order_no) as fee_count
on hdr.order_no = fee_count.order_no
INNER JOIN 
    (select 
        order_no, 
        unit_price 
    from oe_line line with (nolock) 
    where line.inv_mast_uid in (select inv_mast_uid from inv_mast with (nolock) where item_id in ('LARGE-ORDER-1%','LARGE-ORDER-2%', 'LARGE-ORDER-3%'))) as fee_price
ON fee_count.order_no = fee_price.order_no
WHERE
    hdr.projected_order = 'N'
    AND hdr.cancel_flag = 'N'
    AND hdr.delete_flag = 'N'
    AND hdr.completed = 'N'
    AND territory.territory_id = ‘CUSTOMERTERRITORY’
    AND ext_price_calc > 600.00
    AND hdr.carrier_id <> '100004'
    AND fee_count.convenience_count is not null
    AND CASE 
            WHEN (ext_price_calc >= 600.01 and ext_price_calc <= 800) and fee_price.unit_price <>  round(ext_price_calc * -.01,2)
                THEN '-1%: $' + cast(cast(ext_price_calc * -.01 as decimal(18,2)) as varchar(255))
            WHEN ext_price_calc >= 800.01 and ext_price_calc <= 1000 and fee_price.unit_price <>  round(ext_price_calc * -.02,2)
                THEN '-2%: $' + cast(cast(ext_price_calc * -.02 as decimal(18,2)) as varchar(255))
            WHEN ext_price_calc > 1000 and fee_price.unit_price <>  round(ext_price_calc * -.03,2)
                THEN '-3%: $' + cast(cast(ext_price_calc * -.03 as decimal(18,2)) as varchar(255))
            ELSE
                'OK' END <> 'OK'