You'll need to provide more information e.g. what you are rendering (simple box? scene with complex lighting and materials?), are you rendering a single image or multiple frames of an animation, what render engine is being used, render settings on both (I'm assuming you've already compared between the two?), what generation of processor, and so on.
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If you are going to fly by the seat of your pants, expect friction burns.
"I don't know" is the beginning of knowledge, not the end.