Why does my query run so slow? How do I read a diagnostic report? diagnostic report attached

Any CSV is text, and the issue isn't it is a text file, it is that it is a flat file not in a database, like SQL Server or SAP. So any Excel, CSV, TSV, TXT, or other flat file will always run slower because Power Query must do 100% of the work, whereas if the table is in a database, there is a good chance you can optimize your queries to put as much work back on the server. 

For example, I am working on a project  and this table has a few million rows, probably 3.5-4.0 million. This takes about 3 seconds to run, because all of the steps I did in Power Query (a few filters, column selections, etc) were sent back to the SQL Server as a SQL Statement:

select [_].[t0_0] as [Order Type],
    [_].[t1_0] as [Order Number],
    [_].[entered_dt] as [EnteredDateKey],
    [_].[ord_dt] as [OrderDateKey],
    [_].[t2_0] as [OE PO Number],
    [_].[t3_0] as [Customer Number],
    [_].[t4_0] as [AR Reference],
    [_].[tot_sls_amt] as [Total Sales Amount],
    [_].[inv_no] as [Invoice Number],
    [_].[inv_dt] as [InvoiceDateKey],
    [_].[t5_0] as [PaymentDateKey]
from 
(
    select [_].[entered_dt] as [entered_dt],
        [_].[ord_dt] as [ord_dt],
        [_].[tot_sls_amt] as [tot_sls_amt],
        [_].[inv_no] as [inv_no],
        [_].[inv_dt] as [inv_dt],
        ltrim(rtrim([_].[ord_type])) as [t0_0],
        ltrim(rtrim([_].[ord_no])) as [t1_0],
        ltrim(rtrim([_].[oe_po_no])) as [t2_0],
        ltrim(rtrim([_].[cus_no])) as [t3_0],
        ltrim(rtrim([_].[ar_reference])) as [t4_0],
        ltrim(rtrim([_].[user_def_fld_1])) as [t5_0]
    from 
    (
        select [_].[ord_type],
            [_].[ord_no],
            [_].[entered_dt],
            [_].[ord_dt],
            [_].[oe_po_no],
            [_].[cus_no],
            [_].[ar_reference],
            [_].[tot_sls_amt],
            [_].[inv_no],
            [_].[inv_dt],
            [_].[user_def_fld_1]
        from [dbo].[OEHDRHST_SQL] as [_]
        where [_].[cus_no] like 'SFN100%'
    ) as [_]
) as [_]
where (([_].[t1_0] <> '00379675' or [_].[t1_0] is null) and ([_].[t1_0] <> '00379320' or [_].[t1_0] is null)) and ([_].[t1_0] <> '00379674' or [_].[t1_0] is null)

That would take minutes to run, perhaps tens of minutes, if it was a text file because I would have had to load all 3.5M rows in Power Query to filter it down to the 7 records I need in this work in progress.

As to your question on the SUMIFS, I don't know. I'd have to test both ways. I suspect a merge will be faster, but I'd do it both ways to see.