Query runs very slow with Table.SelectRows

Hello Coxy_CMV 

you are here maneuvring a lot with tables and that could it makes slow. However try this more compact code, if it has some impact on the performance and let me know. By the way... how many rows you have in #"WIP"-table?

let
    Source = Table.NestedJoin(#"WIP", {"VIN"}, #"ROColl", {"VIN"}, "ROColl", JoinKind.LeftOuter),
    #"Added Custom" = Table.AddColumn(Source, "Matching Open RO's", each let currentRO = [Job No], currentSales = [Sale Type] in
Text.Combine(Table.SelectRows([ROColl], each [Job Number] <> currentRO & " - " & currentSales)[Job Number],"#(lf)" )),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"ROColl"}),

    #"Changed Type" = Table.TransformColumnTypes(#"Removed Columns",{{"Matching Open RO's", type text}})
in
    #"Changed Type"

 Copy paste this code to the advanced editor in a new blank query to see how the solution works.

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Have fun

Jimmy