The RADIUS timeout in ISE is 120 seconds. The radius timeout that you have to be concerned about is on the network device where defaults are usually around 5 seconds, but I have seen it set as low as 1 second (with issues). The RADIUS authentication timeout has to incorporate the latency in the round trip between the NAD and ISE, the latency between ISE and the external ID store (if used), the time it takes for the external ID store to respond, the time it takes ISE to evaluate authentication and authorization rules, and any other checks such as quarantine status.
Because of the 120 second timeout in ISE, it is unlikely that ISE will be timing out a session before the NADs do.
In ISE you can see a logged response time in the authentication details results, this can be used to determine how loaded a node is.
In the RADIUS protocol settings you can set ISE to flag any authentication step that takes more than 500 ms (up to 10 sec and default is 1 sec). It will log these slow steps in the authentication detail reports.
There is also a Authentication Summary report you can run which will provide response time averages and peaks.
