I have used a halogen oven for years - I found early on that I needed to use the extender ring to prevent scorching as it moves the intense heat further away from the mugs. I find 4 mugs take 20 minutes (though this is currently a bit variable as I think the thermostat on mine is on the way out so will replace the oven in the new year).
Try the extender ring to see if this prevents scorching.
Neil.