How do you change Column Names during Import on CSV (Managing changing column names)

Hi,

I would have thourght this is a common problem but I can't find any thing here.

Enviroment:

Pulling hundreds of CSV files from Sharepoint that have over time had some of the Column Names changed.

Requirement:

Whilst importing each file update any of the old column headings to the current headings.

Issue

I have a solution below but it is VERY VERY VERY slow when it executes and looking at  Resource Monitor Microsoft.Mashup is hammering my memory with hard faults.

Question:

Is there a better way to do this that would be more efficient?

Current Code

    #"Transform to CSV: Create import column" = Table.AddColumn(#"Removed Columns", "Transform File from Binary",
      each let
            Source = Csv.Document([Content],[Delimiter=",", Columns=85, Encoding=1252, QuoteStyle=QuoteStyle.None]),
            #"Promoted Headers" = Table.PromoteHeaders(Source, [PromoteAllScalars=true]),
            #"Rename Column" = Table.RenameColumns(#"Promoted Headers",{
                {"Thick1","S1_THICK1_RATE"},
                {"Thick2","S1_THICK2_RATE"},
                {"Thick3","S1_THICK3_RATE"},
                {"Thick4","S1_THICK4_RATE"},

                {"Length1","S1_LENGTH1_RATE"},
                {"Length2","S1_LENGTH2_RATE"},
                {"Length3","S1_LENGTH3_RATE"},
                {"Length4","S1_LENGTH4_RATE"},

                {"PickFail1","S1_PICKFAIL1_RATE"},
                {"PickFail2","S1_PICKFAIL2_RATE"},
                {"PickFail3","S1_PICKFAIL3_RATE"},
                {"PickFail4","S1_PICKFAIL4_RATE"},

                {"Purge","S1_PURGE_RATE"},

                {"Shutter","S1_SHUTTER_RATE"}
                    },MissingField.Ignore)
                           
        in
        #"Rename Column"),
    #"Expanded Transform File from Binary" = Table.ExpandTableColumn(#"Transform to CSV: Create import column", "Transform File from Binary", {"CustomerID", "ETC ETC ETC"}),
  

Regards

Andrew