Just a quick note… Set-SPEnterpriseSearchService still has the PerformanceLevel option in SP2013, but apart from the crawler component it doesn’t do anything. And in practice changing from Maximum (the default) to Partly Reduced won’t achieve much throttling anyway. Best just leave it at Maximum.
I do not agree. I've set the level to Reduced and I see a big difference in CPU utilization (and crawl time of course).
Interesting... in SharePoint 2013, the PerformanceLevel only applies to the Crawler component and it is doing significantly less than it was in 2010.