Progress column in table dependent on other table

Hi, I have a report with 4 tables representing a hierarchy in ADO.

For each level, I wish to create a progress column which if the item has children, use the average of the children's progress, and if there are no children, simply map the ADO state to a static table i have made with pairs of Status, Score (number between 0 and 1 to indicate percentage).

So for a feature, I would like to check if there are any children. If theres is, take the average of the stories' progress columns and assign that. If there are none, find a match from the StateMapping table, and assign the corresponding progress to that state.

I have been able to do this with both nesting the table of stories, and with fetching matching rows with the Table.SelectRows. However, they both make the performance of my model deteriorate harsly. Am I doing this in a very little computationally efficient manner? If so, how could I do so more efficiently?

In advance, thank you for any help or response. Appending two different formula used for creating new column:

= Table.AddColumn(#"Previous", "Progress", each let
        currentStatus = [State],
        matchingRow = Table.SelectRows(
            StateMapping,
            each [State] = currentStatus
        )
    in
        if Table.RowCount(matchingRow) > 0 and Table.RowCount([Stories]) = 0 then matchingRow{0}[Score]
        else if Table.RowCount([Stories]) > 0 then List.Average([Stories][Progress])
        else 0
)

= Table.AddColumn(#"Previous Step", "Progress", each let
        currentStatus = [State],
        WID = [Work Item Id],
        matchingChildren = Table.SelectRows(
            Tasks,
            each [Parent Work Item Id] = WID
        ),
        matchingRow = Table.SelectRows(
            StateMapping,
            each [State] = currentStatus
        )
    in
        if Table.RowCount(matchingChildren) = 0 and Table.RowCount(matchingRow) > 0 then matchingRow{0}[Score]
        else if Table.RowCount(matchingChildren) > 0 then List.Average(matchingChildren[Progress])
        else 0)