Optimize a MS SQL Scalar FUnction

In the function below...

I am building a #temp table that will have from 20k - 5k records in it
I am then doing a select on that table

Select * from #temp
WHERE 1 = 1
      AND dbo.brkOwnsTimeshareContract(ISNULL(b.ExternalID2, '0')) = 1;

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It seems slow... and I am wondering if I have a better solution or tweak on either my select or the underlying function itself

The dbo.everyware_t_contract table generally has @ 60k records in it

ALTER FUNCTION [dbo].[brkOwnsTimeshareContract]
    (
        @OwnerID INT = NULL
    )
RETURNS TINYINT
AS
    BEGIN

        DECLARE @OwnerBucket TINYINT = 0;

        IF EXISTS (   SELECT ContractID
                      FROM   dbo.everyware_t_contract c
                      WHERE  c.OwnerID = @OwnerID
                             AND (   c.ContractStatus = 'Active'
                                     OR CHARINDEX('Phase', c.ContractStatus) > 0
                                     OR CHARINDEX('Pender', c.ContractStatus) > 0 
									 OR CHARINDEX('Suspense', c.ContractStatus) > 0 
									 OR CHARINDEX('Not Back', c.ContractStatus) > 0 
									 OR CHARINDEX('Presale', c.ContractStatus) > 0 )
                             AND NOT (   CHARINDEX('club', c.ContractType) > 0
                                         OR CHARINDEX('guest', c.ContractType) > 0
                                         OR CHARINDEX('goc co', c.ContractType) > 0
                                         OR CHARINDEX('goc breck', c.ContractType) > 0
                                         OR CHARINDEX('GOC CO $1995', c.ContractType) > 0
                                         OR CHARINDEX('GOC Breck $1295', c.ContractType) > 0
                                         ))
           AND ISNULL(@OwnerID, 0) > 0
            SET @OwnerBucket = 1;

        RETURN @OwnerBucket;
    END;

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