1. It still can if you multitask. For example if you play an online FPS game while you use discord for communication or use a Youtube video as a music source it can still tank your FPS as CPU utilization can easily be maxed out.

2. Generally no. It's most important to have a good airflow, so the general idea is to always never obstruct the airflow of any laptop.

3. Generally Quad Cores in theory should heat up faster once all the cores are utilized, but since a computer system won't be fully utilized all the time, it can be actually be cooler than using a dual core that is always on 100% utilization.
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Hope for the Best. But expect the worst.

Quad Core vs Dual Core processor

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