Will the slave kick in right away and operate properly?
Any PAM version
Yes, it will usually happen in less than 1 minute total. There is a heartbeat every 20 seconds. If no response is received within 20 seconds, the other appliance(s) will assume it is down. They will then check access to their own default gateway to ensure that it is not actually caused by losing its own network connection. Assuming there IS connectivity to the default gateway the remaining appliance(s) will automatically choose a new master and that node will take over responsibility of the VIP.
If speaking ONLY about the 'Primary Site' of a multi-site cluster, this is the same answer.
If a 'Secondary Site' cluster node fails, then it is marked as out of sync until that site is re-synced. The Secondary sites have a 5-minute heart-beat instead of the 20 second heartbeat, allowing for less chance of a short outage causing problems.