Since you did not provide DDL or DML for us to review and evaluate, here are a few general purpose ideas.
If there are
JOINs, ensure that the columns used in the
ON clause are indexed.
If a few non-Key columns are used in
Where clauses, consider adding them to the index as an included column.
Use the SQLParameter Class to pass parameters to SQL statements not concatenated strings with embedded values.
Read Microsoft documentation on
Improving SQL Server Performance[
^]
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