2 Answers. Indexes, clustered or non-clustered,will always slow down inserts as SQL has to maintain both the table and index. This slowdown is in an "absolute" sense and you may not notice it.

Does clustered index slow insert?

Summary: Clustered index blocks inserts. Non clustered works fine.

Does indexing slow down insert?

The number of indexes on a table is the most dominant factor for insert performance. The more indexes a table has, the slower the execution becomes. ... For this reason it has to add the new entry to each and every index on that table. The number of indexes is therefore a multiplier for the cost of an insert statement.