NBN: Why 5GHz WiFi router is critical to get good internet speed

I have Vodafone NBN on 100Mbps/40Mbps plan and below are my two speed test results taken one after the other at around 19:45 (Which is evening peak hours)

Speed test results when connected with 5GHz WiFi

Speed test results when connected to 2.5GHz WiFi

As you can see, 5GHz WiFi is critical to get full download & upload speed on your NBN.