What is the more efficient T-SQL way to query a table with Hierarchical data type

I have a table called Budgetline. It keeps track of budget lines for projects (Grants).

Schema:

BudgetId       int
Amount         decimal
LoginUser      varchar
InsertDate     datetime
GrantPhaseID   int
BudgetChartID  int
Rootdir        hierarchyid
OverHead       decimal

The Amount column is the budgeted amount for a budget line. A budget line can have a sub budgetline. A sub budgetline can have another sub budgetline. Sometimes there can be up to 5 levels.

There is another table TransactionsDetail; it keeps track of budgetline spending

Schema:

TransactionDetailID  int
TransactionID        int
Amount               numeric
ExRateAmount         numeric
TransactionDate      date
BudgetId             int
InsertDate           datetime
OverHead             decimal
Paid                 bit
PaidDate             datetime
LoginUser            varchar

Projects (Grants) have phases. There is another table GrantPhase to keep track of that.

There is another table called BudgetChart. It holds list of Budgetlines. Different projects (Grants) can have same budgetlines.

Below is the complete table-valued function to get the Sub budgetlines (descendants) of a budgetline (Parent).

ALTER FUNCTION [dbo].[getSUBS]
    (@BudgetID INT
     --,@GrantPhaseID INT
    )
RETURNS @Mytable TABLE (CID INT,
                        [COUNT] INT,
                        DESCRIPTION VARCHAR(256),
                        AMOUNT NUMERIC(18,2),
                        SPENT NUMERIC(18,2),
                        BALANCE NUMERIC(18,2),
                        OVERHEAD NUMERIC(18,2)
                        --  BUDGETLIMIT numeric(18,2)
                       )
AS
BEGIN

-- get budgetline root level
declare @BudgetIDrootRevel int
SELECT @BudgetIDrootRevel = RootDir.GetLevel() FROM budgetlines WHERE budgetid = @BudgetID

-- GET GRANTPHASEID
DECLARE @GrantPhaseID int=(
select GrantPhaseID from BudgetLines where BudgetId=@BudgetID
)

DECLARE @RootDir HIERARCHYID
SELECT @RootDir = RootDir FROM budgetlines WHERE budgetid = @BudgetID
insert into @Mytable(
    CID 
,   [COUNT]
,   DESCRIPTION 
,   AMOUNT 
,   SPENT
,   BALANCE
,   OVERHEAD 
--, BUDGETLIMIT
)

SELECT
    BudgetId 
,   ROW_NUMBER() OVER (ORDER BY BudgetID DESC)
,   [Description]
,   dbo.[getBudgetAmount](BudgetLines.BudgetId)  AMOUNT --Sums all transactions made in the TransactionDetails table
,   [dbo].[getBudgetSpent](BudgetId) as SPENT
,   ISNULL((dbo.[getBudgetAmount](BudgetLines.BudgetId)-[dbo].[getBudgetSpent](BudgetId)),0) as BALANCE
,   BudgetLines.OVERHEAD
--, BUDGETLIMIT

FROM BudgetLines INNER JOIN BudgetChart 
ON BudgetChart.BudgetChartID = BudgetLines.BudgetChartID

WHERE RootDir.IsDescendantOf(@RootDir)=1
and GrantPhaseID = @GrantPhaseID
and Rootdir.GetLevel()=(@BudgetIDrootRevel+1)
--AND isBudgetline=1

return ;

end

It works.

• Budgetline table has only 252 records
• TransactionDetails table has only 172 records

My Challenge:

It takes like 10 seconds to return sub budgetlines if that particular budgetline has 3 or more sub levels (descendants).

My Question:

Is there a better way to optimize(rewrite) this function so it can run faster.

Below is how the design looks like. User can see the sub budget lines either by double-clikcing a row or clicking the [Move Down] Button.

How the UI looks

Needless to say: This is my first post on almighty stackoverflow. Sorry If I'v violate any of the community rules. I'm still learning them.