We are tuning a SharePoint 2013 Farm for better search performance today. Microsoft has a great resource with some benchmarks & best practices here:
Keep in mind there are some huge farms listed here. Most SharePoint implementations will fall in the small/medium farm & performance #'s will differ greatly depending on hardware. We have four app servers, two web front ends, and SQL Server on dedicated hardware. We are indexing a LAN, SharePoint Sites, and a couple enterprise applications for a total of close to 40 million documents and crawling on average about 40 documents per second. We are hoping to improve this to closer to 100 documents.