I have an 8x8x7 oven with 7x3100w elements. It is lined with 18g steel and has 4in of mineral wool insulation. The first time I fired it up, it took 1.5 hours to come up to temp. I then installed a fan (6in blade), fired it up again and now it takes 1h 5m to heat to 400. Any ideas on why so long? I would love to have a 15minute heat up, but am afraid that it may not be possible without more elements.