Freeradius Mysql Performance

Hi, Our company is using freeradius as a VPN authentication & authorization system. In worse-case say we would have 1 Million users. Beside scaling our market, we are going to develop an application to analyze users with data mining algorithms. Currently we use a server with the following features: * RAM: 4 GB * Processor: 1x E8400 3.0 GHz For some queries it takes 15 seconds or more to get answer from mysql specially when queries work with radacct table. Any suggestion to improve performance? Regards, Morteza Milani