How to use Table.Buffer to speed queries?

[macfuller]

My queries go against transactional tables with 30-40 columns and millions of rows, and I'd like to speed them up. I see columns by Chris Webb and others talking about how List.Buffer and Table.Buffer can greatly reduce query times, but the M function descriptions are pretty sparse on how to use these.

Is there a rule of thumb about when in a query you should operate on a buffered table vs. the original? For instance, it's fairly typical for my queries to do the following:
1) change column types
2) create new columns
3) filter on source and/or new columns
4) merges
not necessarily in that order.

So at what point would I "hand off" to the buffered table, and then return to the source, to optimize a query like the one below?

let
    SrcFileName = fnGetInternalTableValues("tblQuerySources", 0, 1, "Vouchers"),
    FileSource = fnGetInternalTableValues("tblDefaults", 0, 1, "Data Source Folder Path") & SrcFileName,
    Source = Csv.Document(File.Contents(FileSource),[Delimiter=",",Encoding=1252]),
//    Source = Csv.Document(File.Contents("S:\OFP\SCM Logistics Operations\Sources\3PL_AP_SPEND.csv"),[Delimiter=",",Encoding=1252]),
    PromoteHeaders = Table.PromoteHeaders(Source),
 
    ChangeColumnTypes = Table.TransformColumnTypes(PromoteHeaders,{
        {"Dept ID", type text}, {"Supplier Name", type text}, {"Origin", type text},
        {"Supplier ID", type text}, {"Line Description", type text}, {"Account Description", type text},
        {"Sum of monetary amount", Currency.Type}, {"Account", type text}, {"Business Unit", type text}, 
        {"PO Number", type text}, {"Line Number", Int64.Type}, {"Schedule Number", Int64.Type}, {"PO Distribution Line Number", Int64.Type}, 
        {"Payment Date", type date}, {"Invoice Date", type date}, {"Payment ID", type text},
        {"Payment Amount", Currency.Type}, {"Merchandise Amt", Currency.Type}, {"Sum Freight", Currency.Type}, 
        {"Unit Price", Currency.Type}, {"Quantity", type number}, {"Payment Method", type text},
        {"Discount Amount", Currency.Type}, {"Discount Due Date", type date}, {"Due Date", type date}, {"Voucher Entered Date", type date},
        {"Matched Date", type date}, {"Payment Terms ID", type text}, {"Payment Terms Description", type text},
        {"Voucher Style", type text}, {"Close Status", type text}, {"Post Status", type text},
        {"Invoice Number", type text}, {"Voucher Source", type text}, {"Match Status", type text},
        {"Voucher ID", type text},{"Bank Code", type text},{"Bank Account", type text}, {"Accounting Date", type date},
        {"Match Line Status", type text}, {"Voucher Line Number", Int64.Type}, {"Voucher Approval Status", type text}, {"Voucher Approval Date", type date}, {"Supplier Persistence", type text}
        }),
 
 
    RenameColumns = Table.RenameColumns(ChangeColumnTypes,{
        {"Sum of monetary amount", "Voucher Amount"}, {"Matched Date", "Voucher Match Date"},
        {"Sum Freight", "Freight Charges"}, 
        {"Due Date", "Term Due Date"}, {"Origin", "Invoice Source"},
        {"Line Description", "Item Description"}}),
 
    RemoveDateErrors = Table.RemoveRowsWithErrors(RenameColumns, {"Invoice Date", "Payment Date", 
        "Term Due Date", "Discount Due Date", "Voucher Match Date",
        "Voucher Entered Date", "Accounting Date", "Voucher Approval Date"}),
/*
//    -- For smaller extracts, take only the dates according to the incremental value
//    -- Change the date field below to match the appropriate date field in the source
    DateInc = -30,
    LatestDatesOnly = Table.SelectRows(AddConvFactor, each [Accounting Date] > Date.AddDays(Date.From(DateTime.FixedLocalNow()), DateInc)),
*/
 
/*
    IsOverride = fnGetInternalTableValues("tblDefaults", 0, 1, "Data History Override"),
    OverrideDuration = fnGetInternalTableValues("tblDefaults", 0, 1, "Days History"),
    IfDateOverride = if IsOverride = "Y"
        then Table.SelectRows(RemoveDateErrors, each Date.IsInPreviousNDays([Invoice Date], OverrideDuration))
        else RemoveDateErrors,
*/
 
    IsDateFilter = fnGetInternalTableValues("tblDefaults", 0, 1, "Filter Data by Date"),
    DateFilterStart = Date.From(fnGetInternalTableValues("tblDefaults", 0, 1, "Filter Start Date")),
    DateFilterEnd = Date.From(fnGetInternalTableValues("tblDefaults", 0, 1, "Filter End Date")),
    IfDateFilter = if IsDateFilter = "Y" then
        Table.SelectRows(RemoveDateErrors, each [Voucher Entered Date] >= DateFilterStart and [Voucher Entered Date] <= DateFilterEnd)
    else RemoveDateErrors,
 
 
    AddForeignKey = Table.AddColumn(IfDateFilter, "FK_PO", each [PO Number] & "_" & Number.ToText([Line Number]) & "_" & Number.ToText([Schedule Number]) & "_" & Number.ToText([PO Distribution Line Number]), type text),
    AddPOSource = Table.AddColumn(AddForeignKey, "IsFromPO", each if [PO Number] = "" then "Non-PO" else "PO", type text),
 
    MergeAPPmtTerms = Table.NestedJoin(AddPOSource,{"Payment Terms ID"},tblAPTerms,{"Code"},"NewColumn",JoinKind.LeftOuter),
 
    ExpandAPTermCols = Table.ExpandTableColumn(MergeAPPmtTerms, "NewColumn", {"Group", "Active", "Expected Pay Lag"}, {"Group", "Active", "Expected Pay Lag"}),
    RenamePmtTermCol = Table.RenameColumns(ExpandAPTermCols,{{"Group", "Payment Term Group"}, {"Active", "Active"}}),
 
    MergePwCCategories = Table.NestedJoin(RenamePmtTermCol,{"Account"},tblPwCAccts,{"GL"},"tblPwCAccts",JoinKind.LeftOuter),
 
    ExpandPwCCategories = Table.ExpandTableColumn(MergePwCCategories, "tblPwCAccts", {"PwC Category"}, {"PwC Category"}),
    AddSupplierInitialCol = Table.AddColumn(ExpandPwCCategories, "Supplier Initial", each Text.Range([Supplier Name],0,2), type text),
    AddPaymentKey = Table.AddColumn(AddSupplierInitialCol, "Pmt_Key", each [Bank Code] & "_" & [Bank Account] & "_" & [Payment ID], type text),
    AddVoucherKey = Table.AddColumn(AddPaymentKey, "Vchr_Key", each [Business Unit] & "_" & [Voucher ID], type text),
    AddInvoiceKey = Table.AddColumn(AddVoucherKey, "Invoice_Key", each [Supplier ID] & "_" & [Invoice Number] & "_" & Date.ToText([Invoice Date]), type text)
in
    AddInvoiceKey