Dynamic Allocation % Table

Yeah, it generates the monthly totals in the MonthlyTotal step, then it has to do a Table.SelectRows (a filter) on that based on the current month. So with 5,000 rows, it is doing 5,000 Table.SelectRows. This is what Power Query is not good at. 

However, try this. I did a grouping by month, then did the Table.SelectRows, then re-expanded. So it should cut down the filtering on a per month basis vs every single row.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("jZIxDsMgDEXvwpwI2xgDa7ceoFOUsWO33l/FDIiQBLp9rMe3v+VtM4/XimYxDi2gJSDIj+f3/VkpCwSzL5WhZIEODHJWCSeQOolvIe3mzk6xOtElpIxIzxzHxqjlNIFUMbTQee6iJIwhTSM8YXQkjuNsykSe52c/ZorydUnuPpqHFrpYkn7lsVGJFt3YKGizOGZEq75nuhvRiVL6Awr5kPYf", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"BU Name" = _t, Date = _t, Item = _t, #"Sales €" = _t]),
    #"Changed Type with Locale" = Table.TransformColumnTypes(Source, {{"Date", type date}}, "en-BM"),
    #"Changed Type" = Table.TransformColumnTypes(#"Changed Type with Locale",{{"Sales €", Int64.Type}}),
    #"Grouped Rows" = 
        Table.Group(
            #"Changed Type",
            {"BU Name"}, 
            {
                {
                    "All Rows",
                    each
                        let
                            varTable = _
                        in
                        Table.AddColumn(
                            _,
                            "YTD Revenue",
                            each
                                let 
                                    varCurrentMonthEnd = Date.EndOfMonth([Date]),
                                    varCurrentYear = Date.Year([Date])
                                in
                                List.Sum(
                                    Table.SelectRows(
                                        varTable,
                                        each Date.Year([Date]) = varCurrentYear and [Date] <= varCurrentMonthEnd
                                    )[#"Sales €"]
                                )
                        ), 
                    type table [BU Name=nullable text, Date=nullable date, Item=nullable text, #"Sales €"=nullable number, YTD Revenue = nullable number]
                    }
                }
            ),
    #"Expanded All Rows" = Table.ExpandTableColumn(#"Grouped Rows", "All Rows", {"Date", "Item", "YTD Revenue"}, {"Date", "Item", "YTD Revenue"}),
    #"Removed Other Columns" = Table.SelectColumns(#"Expanded All Rows",{"BU Name", "Date", "YTD Revenue"}),
    #"Removed Duplicates" = Table.Distinct(#"Removed Other Columns"),
    MonthlyTotal = 
        Table.Group(
            #"Removed Duplicates", 
            {"Date"},
            {
                {"YTD Total", each List.Sum([YTD Revenue]), type nullable number}
            }
        ),
    #"Grouped Rows1" = 
        Table.Group(
            #"Removed Duplicates", 
            {"Date"}, 
            {
                {
                    "All Rows", 
                    each
                        let
                            varTable = _
                        in
                        Table.AddColumn(
                            _,
                            "Pct of Total",
                            each
                            let
                                varCurrentMonthEnd = Date.EndOfMonth([Date])
                            in
                                [YTD Revenue] /
                                Table.SelectRows(
                                    MonthlyTotal,
                                    each [Date] = varCurrentMonthEnd
                                )[YTD Total]{0}
                            ),
                            type table [BU Name=nullable text, Date=nullable date, YTD Revenue=nullable number, Pct of Total=nullable Percentage.Type]}
            }
        ),
    #"Expanded All Rows1" = Table.ExpandTableColumn(#"Grouped Rows1", "All Rows", {"BU Name", "Pct of Total"}, {"BU Name", "Pct of Total"}),
    #"Pivoted Column" = Table.Pivot(#"Expanded All Rows1", List.Distinct(#"Expanded All Rows1"[#"BU Name"]), "BU Name", "Pct of Total", List.Sum)
in
    #"Pivoted Column"

It returns this.

Kudos are appreciated on any posts that helped.
Let me know if this gets you there. If it does't though, I am not sure Power Query will work for you in this case.