In NLB unicast mode Network card's MAC address gets replaced by a virtual MAC which prevents the servers from connecting to each other if only one network card is installed on WFE/Query machines. By default WFE may reroute the search request to any of the Query servers available in the Farm (Including itself) and the above setting forces the Search service to use the local Query server and keeps retrying the other WFE/Query servers in 30 seconds, 60 seconds,etc. As a result you should see a similar warning in the Machine's Event Log:
Retry of query machine '[MachineName]' has failed with error: The network path was not found. 0x80070035. It will be retried again in 900 seconds. Component: 8187f82b-d5df-47e8-b26a-a19bca2588c9
Adding a second Network card should resolve this issue.