During a current project i learned that cisco supports a great feature that i missed currently (and a long time before) at all EOS and EXOS switches !
Marking a unreachable RADIUS Server as DEAD and skip them at all following Authenication session. If the RADIUS Server is accessable again use it again.
EXOS and EOS have to beat every Authentication session through the configured timeout and re-tries to use the failover system. If the primary RADIUS failed the complete Authentication process needs more time because of waiting for timeouts....
Cisco achieve this with a "RADIUS Server dead" status.
I think EXOS (and maybe EOS) should be enhanced with such a feature.