Optimise Query for Calculating Number of Business Days

The key to performance is to only calculate this column once instead of once for each row of your date table.

My approach would be to add an index column after filtering out weekends and holidays and then merge this back with the original dates.

let
    StartDate = #date(2019, 3, 1),
    EndDate = Date.From(DateTime.LocalNow()),
    ListDates = List.Dates(StartDate, Number.From(EndDate - StartDate), #duration(1, 0, 0, 0)),
    ListToTable = Table.FromList(ListDates, Splitter.SplitByNothing(), {"Date"}),
    DateTable = Table.TransformColumnTypes(ListToTable, {{"Date", type date}}),
    RemoveDays = Table.SelectRows(DateTable, each Date.DayOfWeek([Date], 1) < 5 and
                                                  not List.Contains(#"bank-holidays-dl",
    AddIndex = Table.AddIndexColumn(RemoveDays, "Index", 1, 1, Int64.Type),
    MergeQueries = Table.NestedJoin(DateTable, {"Date"}, AddIndex, {"Date"}, "RemovedDays", JoinKind.LeftOuter),
    ExplandIndex = Table.ExpandTableColumn(MergeQueries, "RemovedDays", {"Index"}, {"WorkingDaysSinceStart"}),
    SortRows = Table.Sort(ExplandIndex,{{"Date", Order.Ascending}}),
    FillDown = Table.FillDown(SortRows, {"WorkingDaysSinceStart"})
in
    FillDown