Well although previously we see how OP the mc is, its good to have him struggle with current world's unique magical properties. It would be boring if the system of both world is too identical.
He is not nerfed, he is testing his magic compared to the magic of this world, @nfzeta and yes, he not from this world nor he is the hero or use the magic from this world, and he because of the lack of the spirits he probably don't deal that much damage.
the demons just have super high magic resistance. even his powerful flame took a minute to kill the demon, but when used on other it should only take a second. (or i just remembered wrong cause i read fast ?)
It's completely identical, as with all worlds in this series seems to be. People just develop different techniques to use it. It's an issue to do with the demon's nature that he's overthinking his way into missing. Something that's rare to encounter where he comes from and thus rarely accounted for in spell formulations.