Is this a SQL Server question or an Oracle question? Somehow this shows up in the (Oracle) PL\SQL zone, but the terms you use in your question title (Clustered and Non-clustered Index) are SQL Server-only terms.
if your table has high data insert delete operations then non-clustered indexes need to be reorginized and clustered indexes to be rebuild. If not they may slow down the performance. How often do you do this operations or did you do?
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