When all configuration and policies are stored in RAM, the server is capable of performing 10's of 1000's of PAP authentications per second on commodity hardware. Specific performance capabilities depend on a number of factors, including:

In general, any commodity hardware will be sufficient for systems up to a few hundred thousand users. For a larger number of users, issues such as high-availability and fail-over will control the choice of hardware. Once sufficient hardware has been deployed to meet high-availability requirements, RADIUS performance will not be an issue.

Some commercial servers claim surprisingly high numbers for their performance. In our tests, the security overhead of RADIUS (MD5 hashes) can account for up to fifty percent (50%) of the time spent processing a packet. Any claims that a product has more than double the performance of a tuned FreeRADIUS installation are best viewed with skepticism.

The choice of database can strongly affect server performance. Each accounting packet may result in one or more write requests to a database. The database performance then limits how many accounting packets can be processed, and therefore limits the performance of the RADIUS server. Many database cannot perform more than a few hundred to a few thousand writes per second. This database limitation in turn lowers RADIUS performance to about the same number of packets per second.

The best way to determine the performance of a server is via testing. FreeRADIUS comes with a tool called radclient, which can be used for basic peformance testing. A third party tool called RadPerf is also available. It uses the FreeRADIUS libraries to implement the RADIUS portion, and then builds more complex functionality on top of that. It can be used to simulate user logins, and can auto-generate accounting packets for user sessions.