Search performance will be improved if more indexers are added in the indexer cluster. To get more out of it you can tune your search queries to use streaming commands before non-streaming commands to improve parallel processing so that searches are sent to the indexers to process as much of the search as possible on the indexers.
if the data is balanced accordingly, your search performance will be better as more indexers generate more IOPS
otherwise, the performance will be based on your worse performing indexer both will also always depend on and your SH constrains (CPU, Memory)
Currently we just have one search head over indexer cluster with 2 peers on it, we were asked to add additional standalone indexer have data of another environment to the same search head.
Will this cause any performance impact as search head has more splunk indexers to search from.