Slow transformation - List.Buffer or Table.Buffer as possible solution?

Use the below code to overcome performance bottleneck. (For improving performance further, you can use my formula for conversion. It will avoid overhead of calling a function and is also shorter. In below code, I have used your function)

let
    Source = Folder.Files("D:\Downloads\New folder"),
    #"Filtered Hidden Files1" = Table.SelectRows(Source, each [Attributes]?[Hidden]? <> true),
    #"Invoke Custom Function1" = Table.AddColumn(#"Filtered Hidden Files1", "Transform File", each #"Transform File"([Content])),
    #"Renamed Columns1" = Table.RenameColumns(#"Invoke Custom Function1", {"Name", "Source.Name"}),
    #"Removed Other Columns1" = Table.SelectColumns(#"Renamed Columns1", {"Source.Name", "Transform File"}),
    #"Expanded Table Column1" = Table.ExpandTableColumn(#"Removed Other Columns1", "Transform File", Table.ColumnNames(#"Transform File"(#"Sample File"))),
    #"Changed Type" = Table.TransformColumnTypes(#"Expanded Table Column1",{{"Source.Name", type text}, {"Order ID", type text}, {"Product Name", type text}, {"Discount", type number}, {"Sales", Int64.Type}, {"Profit", Int64.Type}, {"Quantity", Int64.Type}, {"Category", type text}, {"Sub-Category", type text}, {"Job Ref", type text}}),
    #"Removed Columns" = Table.RemoveColumns(#"Changed Type",{"Source.Name"}),
    #"Added Custom" = Table.AddColumn(#"Removed Columns", "Short Job Ref", each Text.Start([Job Ref],6)),
	BuffList = List.Buffer(#"Added Custom"[Short Job Ref]),
   ListCount = List.Count(BuffList),
    GenHex2Dec = List.Generate(()=>[x=fnHex2Dec(BuffList{0}),i=0], each [i]<ListCount, each [i=[i]+1, x=fnHex2Dec(BuffList{i})], each [x]),
    Result = Table.FromColumns(Table.ToColumns(#"Added Custom")&{GenHex2Dec},Table.ColumnNames(#"Added Custom")&{"fnHex2Dec"}),
    #"Removed Columns1" = Table.RemoveColumns(Result,{"Job Ref", "Short Job Ref"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Columns1",{{"fnHex2Dec", "Job Ref"}}),
    #"Merged Queries" = Table.NestedJoin(#"Renamed Columns", {"Order ID"}, tblDesc, {"Order ID"}, "tblDesc", JoinKind.LeftOuter),
    #"Expanded tblDesc" = Table.ExpandTableColumn(#"Merged Queries", "tblDesc", {"lon", "lat"}, {"lon", "lat"})
in
    #"Expanded tblDesc"