The error message "Your Search Results are not optimized. If the results are not what you expected, please try again later." means that the dedicated search service was unable to handle the request.
The results you may see were instead returned by the database (in a different, less optimal ordering than those that would be returned by the search service). Though this happens infrequently, this fall back search process is in place to prevent no results from being returned from a search. We apologize for any inconvenience.
This is typically a temporary situation but may also be indicative of or caused by a performance degradation. Check to see whether your instance is reporting any issues on the Trust site that may account for the search behavior you're seeing.
Check Trust frequently for updates or new reports if you continue to see this message for an extended period of time. If the behavior persists and there are no reports of a performance issue on your instance please log a case with Salesforce Support for further investigation.