Laptops aren't the best for gaming and one of the reasons why is that they tend to run very hot, since they don't have as much space to properly manage their heat. So the heat isn't unusual. It's quite possible that this heat is causing the performance drops. Your system might be scaling itself down when it's getting too hot, in order to prevent damage from overheating.
If you're serious about wanting to play demanding games on a laptop, it's not an unnecessary luxury to get yourself one of those laptop cooling pads, which you can place under your laptop to blow cool air into the system, helping with temperature management a bit by increasing airflow. Barring that, simply making sure the laptop's vents aren't blocked by anything will help a little as well. For example, if the laptop is blowing air out of the bottom, putting it on a flat surface like a table might be restricting the airflow, so placing something underneath it to keep the vents clear of any surface could help.