Performance tweaking and testing.

Hi all, We have 1500 customers connected to our PPPoE servers, to autenticate we have 2 freeradius servers connected to a mssql server. The radius servers don't autenticate fast enough, our PPPoE server starts to hang when there are to many radius autentications idle/waiting for a answer from the radius server. Does anyone got a tip on how to improve performance on the radius servers? And how to test preformance?