Will the performance of Elasticsearch impacted while re-index?

We are using Change Data Capture (CDC) to sync up data between my source database to elasticsearch.
Should i do re-index, is it needed?
If I do re-index will it impact the performance of elastic users?
if the performance is hit, then what is the work-around?

Apologies, if there is any other similar thread on the same topic. Because I couldn't find that thread.

In what context? The MSSQL one?

Indexing does impact the cluster as it takes up resources, to what extent depends on your cluster though. If you don't want that to happen then you should scale out.

Yes, in MSSQL o Elasticsearch. So, first place re-index is not needed although I do updates to ES using NEST code?