Tunning Guide

Main Server

If you have a large user base and/or many authentication requests try using a scalable authentication mechanism like ldap or sql.
Enable noatime on all the freeradius log files or better yet on the freeradius log directory.
Always use the latest cvs version. It will probably contain a few fixes and enhancements.
Always try to use the least modules possible. In particular if you can avoid it do not use the detail and radwtmp (files) modules. They will slow down your accounting.
Use the users file to only set default profiles. Do not place any users there. Keep it as small as possible. Always set default attributes in the users file and don’t fill the user entries in ldap/sql with default values. In general the ldap/sql user profiles should contain user attributes only in special user cases.
Tune thread pool parameters to match your size requirements. Set max_requests_per_server to zero to avoid server thread restarts.
Enlarge the timeout (10 secs) and retries (5-7) in the access servers for accounting. That way you won’t lose any accounting information.
Use well tuned Fast Ethernet connections to minimize latency.
freeradius is multi threaded and i/o bound. That means you should use the latest OS kernels/patches for improved multi processor and network performance.

Try to maximize caching in the ldap server. In particular always enable indexing of the uid attribute (equality index) and the cn attribute (equality index - the cn attribute is used to search for groups). Make the ldap server entry/directory cache memory sizes as large as possible. In general try allocating as much memory as you can afford to your ldap server.
Put default profiles in ldap. User entries should only contain non standard values in order to remain small and maximize the gains of caching the user default/regular profiles.
Enable group caching in groups.

Use the sql module in the session section instead of the radutmp module. It works much quicker.
Create a multi column index for the (UserName, AcctStopTime) attributes especially if you are using sql for double login detection.
If you are using mysql and you do a lot of accounting try using InnoDB for the radacct table instead of MyISAM (this should be the default in all schemas)
Add AcctUniqueId in the Acct-Status-Type = Stop query. Especially if you have a lot of access servers or your NAS does not send very random Session-Ids. That way you will always have one candidate row to search for, instead of all the rows that have the same AcctSessionId.