We'd probably need to see the workbook in order to troubleshoot.
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You can read about efficient calculations here: Create Efficient Calculations - Tableau
You should be able to create a Reference Line using a Table Calculation. Because Table Calculations work with the cache, they are much faster than Basic Expressions and LODs.
Hope this helps. If you have any further question do not hesitate on asking. If this answer is helpful or answer your question please mark it as helpful/correct so other users can refer to it.
You are welcome to upload your workbook as a twbx so I can have a look at it.