@iVaibhav 

Hey,

I think what you need is load balance, not link backup. Link backup is designed to work when primary WAN is completely down. This feature is based on online detection. Every two minutes, the router will ping the DNS server of the WAN ports to detect whether the WAN is online. If the primary WAN is detected as offline, the router will switch to the backup WAN and continue to work.

However, if the issue is just on the poor performance, the ping of online detection will just get latency, not completely timeout or no response, so the WAN will still be regarded as online, and won't switch to the backup WAN.

It's hard to define how long the latency should consider as poor performance. If the parameter is set as a short time, the online detection will be very sensitive, and the network will jump between two WAN connections back and forth, and during the switching time, you will experience no internet in a short time. If the parameter is set as longer, I think it won't have apparent help in this matter, and may even lead to more complicated problems.

This is just my opinion. Others may feel differently.