Sharepoint Search service uses SQL Server FTS. Based on results listed

http://technet.microsoft.com/en-us/library/cc262574(office.12).aspx

you should be able to do 20 requests per second on a 15 million item library with one server.

20 RPS is quite a bit, even for a public website. If your getting that many, you probably got a datacenter or two and a team of DBAs