My queries are running so slowly!

Thank you so much. I'm still struggling to make it work. 7 hours so far today! Will keep you posted. 

I'm losing my pateince this this!!!! Where am I going wrong?!! Everything I'm trying seems to take ages to run. I also need to replace all the nulls in the Running Total column with zeros. I tried pulling in Replace Values, but that took ages to run.

fxSecRunningTotal

(Table as table, SortColumn as text, AmountColumn as text) =>

let
/* Debug parameters
Table = #"4c - Sec Data Table", 
SortColumn = "CaseNo", 
AmountColumn = "Sec Avg Activity SEI",
*/

// Sort table and buffer it
    Sorted = Table.Buffer(Table.AddIndexColumn(Table.Sort(Table,{{SortColumn, Order.Ascending}}), "Index",1,1)),
// Select the Columns
    SelectColumns = Table.SelectColumns(Sorted, {SortColumn, AmountColumn, "Index"}),
// Extract Amount column and buffer it
    ExtractAmountColumn = List.Buffer(Table.Column(SelectColumns, AmountColumn)),
// Calculate a list with all running Totals
    RunningTotal = List.Skip(List.Generate( ()=> [ListItem=0, Counter=0]
                                            ,each [Counter] <= List.Count(ExtractAmountColumn)
                                            ,each [ ListItem = ExtractAmountColumn{[Counter]}+[ListItem],
                                                    Counter = [Counter]+1
                                                   ]
                                            ),1),
    ConvertedTable = Table.FromList(RunningTotal, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    ExpandedColumn = Table.ExpandRecordColumn( ConvertedTable, "Column1", {"ListItem", "Counter"}, {"ListItem", "Counter"}),
    MergedQueries = Table.NestedJoin(Sorted,{"Index"}, ExpandedColumn,{"Counter"},"Expanded Column1",JoinKind.LeftOuter),
    Expand = Table.ExpandTableColumn( MergedQueries, "Expanded Column1", {"ListItem"}, {"RunningTotal"}),
    #"Removed Columns" = Table.RemoveColumns(Expand,{"Index"}),
    #"Changed Type" = Table.TransformColumnTypes(#"Removed Columns",{{"RunningTotal", type number}})
in
    #"Changed Type"

1 - Data Table

let
    Source = Table.NestedJoin(#"0d - Staging", {"Activity"}, #"0e - AVG", {"Activity"}, "2 - AVG", JoinKind.LeftOuter),
    #"Expanded 2 - AVG" = Table.ExpandTableColumn(Source, "2 - AVG", {"Ops", "MT", "Sec"}, {"Ops", "MT", "Sec"}),
    #"Renamed Columns" = Table.RenameColumns(#"Expanded 2 - AVG",{{"Ops", "Ops Avg Activity SEI"}, {"MT", "MT Avg Activity SEI"}, {"Sec", "Sec Avg Activity SEI"}}),
    
    //Sort rows by case number and activity order, then remove top two rows as blank
    #"Sorted Rows" = Table.Sort(#"Renamed Columns",{{"CaseNo", Order.Ascending}, {"Activity Order", Order.Ascending}}),
    #"Inserted CaseNoActOrderID" = Table.AddColumn(#"Sorted Rows", "CaseNoActOrderID", each Text.Combine({[CaseNo], Text.From([Activity Order], "en-GB")}, ":"), type text),
    #"Reordered Columns" = Table.ReorderColumns(#"Inserted CaseNoActOrderID",{"AC", "CaseNo", "HighestCaseType", "FCCCreatedDate", "Activity", "Activity Order", "CaseNoActOrderID", "Ops SEI Value", "MT SEI Value", "Sec SEI Value", "Ops Avg Activity SEI", "MT Avg Activity SEI", "Sec Avg Activity SEI"}),
    
    //Add in total of the average SEI values for the case, for that division
    #"Added Ops Tot Avg SEI" = Table.AddColumn(#"Reordered Columns", "Ops Total Avg SEI", each let _item = [CaseNo] in List.Sum(Table.SelectRows(#"Reordered Columns", each [CaseNo] = _item)[#"Ops Avg Activity SEI"])),
    #"Added MT Tot Avg SEI" = Table.AddColumn(#"Added Ops Tot Avg SEI", "MT Total Avg SEI", each let _item = [CaseNo] in List.Sum(Table.SelectRows(#"Reordered Columns", each [CaseNo] = _item)[#"MT Avg Activity SEI"])),
    #"Added Sec Tot Avg SEI" = Table.AddColumn(#"Added MT Tot Avg SEI", "Sec Total Avg SEI", each let _item = [CaseNo] in List.Sum(Table.SelectRows(#"Reordered Columns", each [CaseNo] = _item)[#"Sec Avg Activity SEI"]))
in
    #"Added Sec Tot Avg SEI"

 2 - Max Activity Table

let
    Source = #"1 - Data Table", 
    
    #"Buffer Table" = Table.Buffer(Source),
    #"Grouped Rows" = Table.Group(#"Buffer Table", {"CaseNo"}, {{"MaxActivity", each List.Max([Activity Order]), type nullable number}})
    
in
    #"Grouped Rows"

3c - Sec OverUnder Index

let
    Source = #"1 - Data Table",
    #"Buffered Table" = Table.Buffer(Source),
 
//  Calculate Running Total
    #"Grouped Rows" = Table.Group(#"Buffered Table", {"CaseNo"}, {{"All", each _, type table [Column1=nullable text, Sec Avg Activity SEI=nullable number, Date=nullable date]}}),
    #"Invoked Custom Function" = Table.AddColumn(#"Grouped Rows", "fxSecRunningTotal", each fxSecRunningTotal([All], "CaseNo", "Sec Avg Activity SEI")),
    #"Expanded fxSecRunningTotal1" = Table.ExpandTableColumn(#"Invoked Custom Function", "fxSecRunningTotal", {"AC", "CaseNo", "HighestCaseType", "FCCCreatedDate", "Activity", "Activity Order", "CaseNoActOrderID", "Sec SEI Value", "Sec Avg Activity SEI", "Sec Total Avg SEI", "Sec Avg Running Total"}, {"AC", "CaseNo.1", "HighestCaseType", "FCCCreatedDate", "Activity", "Activity Order", "CaseNoActOrderID", "Sec SEI Value", "Sec Avg Activity SEI", "Sec Total Avg SEI", "Sec Avg Running Total"}),

//  Replace Nulls with zeros
    #"Replaced Value" = Table.ReplaceValue(#"Expanded fxSecRunningTotal1",null,0,Replacer.ReplaceValue,{"Sec Avg Running Total"}),

//  Remove 'All' Column as not required
    #"Remove All Column" = Table.RemoveColumns(#"Replaced Value",{"All"}),

//  Change data types to numbers
    #"Changed Type" = Table.TransformColumnTypes(#"Remove All Column",{{"Sec SEI Value", type number}, {"Sec Avg Activity SEI", type number}, {"Sec Total Avg SEI", type number}, {"Sec Avg Running Total", type number}}),

//  Calculate over/under SEI
    #"Add OverUnder" = Table.AddColumn(#"Changed Type", "Sec OverUnder", each if [Sec Avg Running Total] > [Sec SEI Value] then "Over" else if [Sec Avg Running Total] < [Sec SEI Value] then "Under" else "Equal"),

//  Group OverUnder by Case
    #"Grouped Rows1" = Table.Group(#"Add OverUnder", {"CaseNo.1", "Sec OverUnder"}, {{"Count", each _, type table [CaseNo=nullable text, AC=text, CaseNo.1=text, HighestCaseType=text, FCCCreatedDate=datetime, Activity=text, Activity Order=number, CaseNoActOrderID=text, Sec SEI Value=nullable number, Sec Avg Activity SEI=nullable number, Sec Total Avg SEI=nullable number, Sec Avg Running Total=nullable number, Sec OverUnder=any]}}),

//  Add index value to OverUnder with Case
    #"Added Custom1" = Table.AddColumn(#"Grouped Rows1", "Custom", each Table.AddIndexColumn([Count], "Index1", 1)),
    #"Expanded Custom" = Table.ExpandTableColumn(#"Added Custom1", "Custom", {"CaseNo", "CaseNoActOrderID", "Index1"}, {"Custom.CaseNo", "Custom.CaseNoActOrderID", "Custom.Index1"}),

//  Remove unrequired columns
    #"Removed Columns" = Table.RemoveColumns(#"Expanded Custom",{"Sec OverUnder", "Count", "Custom.CaseNo"})
in
    #"Removed Columns"

4c - Sec Data Table

let
    Source = #"1 - Data Table",
    #"Buffered Table" = Table.Buffer(Source),

//  Running Total
    #"Grouped Rows" = Table.Group(#"Buffered Table", {"CaseNo"}, {{"All", each _, type table [Column1=nullable text, Sec Avg Activity SEI=nullable number, Date=nullable date]}}),
    #"Invoked Custom Function" = Table.AddColumn(#"Grouped Rows", "fxSecRunningTotal", each fxSecRunningTotal([All], "CaseNo", "Sec Avg Activity SEI")),
    #"Expanded fxSecRunningTotal1" = Table.ExpandTableColumn(#"Invoked Custom Function", "fxSecRunningTotal", {"AC", "CaseNo", "HighestCaseType", "FCCCreatedDate", "Activity", "Activity Order", "CaseNoActOrderID", "Sec SEI Value", "Sec Avg Activity SEI", "Sec Total Avg SEI", "Sec Avg Running Total"}, {"AC", "CaseNo.1", "HighestCaseType", "FCCCreatedDate", "Activity", "Activity Order", "CaseNoActOrderID", "Sec SEI Value", "Sec Avg Activity SEI", "Sec Total Avg SEI", "Sec Avg Running Total"}),
    #"Remove All Column" = Table.RemoveColumns(#"Expanded fxSecRunningTotal1",{"All"}),

//  Calculate over/under SEI
    #"Added Over/Under" = Table.AddColumn(#"Remove All Column", "Sec OverUnder", each if [Sec Avg Running Total] > [Sec SEI Value] then "Over" else if [Sec Avg Running Total] < [Sec SEI Value] then "Under" else "Equal"),
    #"Changed Type" = Table.TransformColumnTypes(#"Added Over/Under",{{"Sec Total Avg SEI", type number}, {"Sec Avg Running Total", type number}}),
    
//  Calculate remaining balance
    #"Calculate Remaining Balance" = Table.AddColumn(#"Changed Type", "Sec SEI Balance", each [Sec SEI Value] - [Sec Avg Running Total], type number),
    
//  Calculate Final Activity and display in new column
    #"Max Activity Find" = Table.NestedJoin(#"Calculate Remaining Balance", {"CaseNo"}, #"2 - Max Activity Table", {"CaseNo"}, "Max Activity Table", JoinKind.LeftOuter),
    #"Max Activity Display" = Table.ExpandTableColumn(#"Max Activity Find", "Max Activity Table", {"MaxActivity"}, {"MaxActivity"}),

//  Calculate updated Running Total
    #"Grouped Rows2" = Table.Group(#"Max Activity Display", {"CaseNo"}, {{"All", each _, type table [Column1=nullable text, Sec Avg Activity SEI=nullable number, Date=nullable date]}}),
    #"Invoked Custom Function2" = Table.AddColumn(#"Grouped Rows2", "fxSecRunningTotal", each fxSecRunningTotal([All], "CaseNo", "Sec Avg Activity SEI")),
    #"Expanded fxSecRunningTotal2" = Table.ExpandTableColumn(#"Invoked Custom Function2", "fxSecRunningTotal", {"AC", "CaseNo", "HighestCaseType", "FCCCreatedDate", "Activity", "Activity Order", "CaseNoActOrderID", "Sec SEI Value", "Sec Avg Activity SEI", "Sec Total Avg SEI", "Sec Avg Running Total2"}, {"AC", "CaseNo.1", "HighestCaseType", "FCCCreatedDate", "Activity", "Activity Order", "CaseNoActOrderID", "Sec SEI Value", "Sec Avg Activity SEI", "Sec Total Avg SEI", "Sec Avg Running Total2"}),

//  Remove 'All' column as not required
    #"Remove All Column2" = Table.RemoveColumns(#"Expanded fxSecRunningTotal2",{"All"}),

//  Calculates the additional value to force final number to balance back to total SEI
    #"Calculate additional value for max activity" = Table.AddColumn(#"Remove All Column2", "Sec Over SEI Value", each if [Sec OverUnder] = "Over" then ([Sec Avg Running Total2] + [Sec SEI Balance]) - [Sec Avg Activity SEI] else 0),
    
//  Indexes activities within a case and whether they are over or under the SEI for the case. This is to establish additional apportionment values
    #"Added Index to Case and Over/Under" = Table.NestedJoin(#"Calculate additional value for max activity", {"CaseNoActOrderID"}, #"3c - SecOverUnder Index", {"Custom.CaseNoActOrderID"}, "3c - SecOverUnder Index", JoinKind.LeftOuter),
    #"Expanded 3c - SecOverUnder Index" = Table.ExpandTableColumn(#"Added Index to Case and Over/Under", "3c - SecOverUnder Index", {"Custom.Index1"}, {"Sec OverUnder Index"}),
    
//  SEI value if running total is greater than the total SEI for that case
    #"Handling of SEI when running total is greater than total value" = Table.AddColumn(#"Expanded 3c - SecOverUnder Index", "Custom", each if [Sec OverUnder] = "Over" and [Sec OverUnder Index] = 1 then [Sec Avg Activity SEI] + [Sec SEI Balance] else 0),
    #"Renamed Columns1" = Table.RenameColumns(#"Handling of SEI when running total is greater than total value",{{"Custom", "Sec Over SEI New Value"}}),
    
//  SEI value if running total is under and it's the final activity
    #"Handling of final activity if running total is under total value" = Table.AddColumn(#"Renamed Columns1", "Custom", each if [Sec OverUnder] = "Under" and [Activity Order] = [MaxActivity] then [Sec Avg Activity SEI] + [Sec SEI Balance] else 0),
    #"Renamed Columns2" = Table.RenameColumns(#"Handling of final activity if running total is under total value",{{"Custom", "Sec Under SEI New Value"}}),
    
//  Final Sec SEI Value
    #"Add Sec Final SEI" = Table.AddColumn(#"Renamed Columns2", "Sec Final SEI Value", each if [Sec OverUnder] = "Under" and [Activity Order] <> [MaxActivity] then [Sec Avg Activity SEI] else if [Sec OverUnder] = "Under" and [Activity Order] = [MaxActivity] then [Sec Under SEI New Value] else if [Sec OverUnder] = "Over" and [Sec OverUnder Index] = 1 then [Sec Over SEI New Value] else if [Sec OverUnder] = "Over" and [Sec OverUnder Index] <> 1 then [Sec Over SEI New Value] else 0)
in
    #"Add Sec Final SEI"