Yes, OnGuard can take 1-3 minutes to complete health checks.
It depends on the application's installed on the machine and health classes configured.
Health collection is done periodically by Backend Service (ClearPass Agent Controller) as soon as machine starts even if network is not connected.
Back end Service already have logic to periodically collect health and returned cached health status if OnGuard Agent (Front end) requests for health again.
Connecting first time after machine is started takes longer than subsequent retries because sometimes OnGuard have to send 2 requests to CPPM Server. This happens when any of the following health classes are configured:
4. Windows Hotfixes
The reason for 2 requests is that in first request OnGuard does not know what all Registry keys, Services etc to check for so it sends empty list.
If any of above mentioned health classes are configured, then the response from Server contains list of Registry Keys, Services etc to monitor.
In 2nd request, OnGuard sends the status of these monitored Registry Keys, Services etc.
After 1st request, Backend Service remembers these lists in its Memory and uses them in subsequent requests.
These lists are cleared every time Back end Service is restarted.
Note that health collection time depends on Machine Configuration (Processor Speed, RAM etc) and its current state (load on the server) also.