Facetime overloading my computer's scheduler
I believe your MacBook is overheating, which will lead to throttling (automatic downclocking) of your CPU and GPU. While most Intel-based MacBooks do suffer from that to a certain extent, it shouldn't be triggered by a 15-minute FaceTime call.

When using Activity Monitor.app, make sure to have View → All Processes enabled, so system processes are shown as well.

You may use the app iStat Menus or Hardware Monitor to check the temperatures of your MacBook.

It's possible that your MacBook is just dusty, which prevents proper airflow and therefore limits cooling. Here's a link to a video showing a (very thorough) cleaning process: Fixing My Filthy Overheating MacBook Pro. It's usually sufficient to just (carefully!) vacuum the dust from the fans though.