Question about Panasonic NN SD 27 HSGTG

I owned the equivalent Panasonic Inverter 27L and 1000w with grill bought 14 years ago. It heated a cup of water in 43 seconds. Now I have another model of another brand 800W (non inverter) 20L with grill that heats the same cup in 65 seconds. Approximately. Having said that, I used the grill+waves to defrost bread (some brands advise against defrosting bread with the grill because of the risk of fire).