Optimize performance at Un-Cumulate calculation

assume your data is sorted by Datum from smallest to largest. then try this code

=Table.FromRecords(List.Accumulate(Table.ToRecords(#"Removed Duplicates"),{{},0},(x,y)=>{x{0}&{y&[Wert=if y[Kennzahl] = "Menge" then y[KummWert] -x{1} else y[KummWert]]},if y[Kennzahl] = "Menge" then y[KummWert] else x{1}}){0})

I forgot to include the final steps of taking the difference.

Here's what the result looks like (highlighted columns are new, non-highlighted are starting data):

Here's the full code for this example:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjJV0lEyBGLf1Lz0VCDtCOKbKsXqRCsZGmGRM4LImVqA2EhyTkBsYgCWMzbBImcEkTOxxCJnCJSLBQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, Jahr = _t, Kennzahl = _t, Region = _t, Datum = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}, {"Jahr", Int64.Type}, {"Kennzahl", type text}, {"Region", type text}, {"Datum", Int64.Type}}),
    #"Sorted Rows1" = Table.Sort(#"Changed Type",{{"Jahr", Order.Ascending}, {"Region", Order.Ascending}, {"Datum", Order.Ascending}}),
    #"Added Index0" = Table.AddIndexColumn(#"Sorted Rows1", "Index0", 0, 1, Int64.Type),
    #"Added Index1" = Table.AddIndexColumn(#"Added Index0", "Index1", 1, 1, Int64.Type),
    #"Merged Queries" = Table.NestedJoin(#"Added Index1", {"Index0", "Jahr", "Kennzahl", "Region"}, #"Added Index1", {"Index1", "Jahr", "Kennzahl", "Region"}, "Added Index1", JoinKind.LeftOuter),
    #"Expanded Added Index" = Table.ExpandTableColumn(#"Merged Queries", "Added Index1", {"Datum"}, {"PrevDatum"}),
    #"Sorted Rows2" = Table.Sort(#"Expanded Added Index",{{"Jahr", Order.Ascending}, {"Region", Order.Ascending}, {"Datum", Order.Ascending}}),
    #"Added Custom" = Table.AddColumn(#"Sorted Rows2", "UncumDatum", each [Datum] - (if [PrevDatum] = null then 0 else [PrevDatum]), Int64.Type)
in
    #"Added Custom"