Facetime overloading my computer's scheduler Running Mac OSX High Sierra 10.13.6.
Many times when I'm on a facetime call, and when doing other video things, my whole computer gets hangy about 15 minutes in.  I'll throw in specs on my machine:
Model Name: MacBook Pro
Model Identifier: MacBookPro9,2
Processor Name:   Intel Core i7
Processor Speed:  2.9 GHz
Number of Processors: 1
Total Number of Cores:    2
L2 Cache (per Core):  256 KB
L3 Cache: 4 MB
Memory:   8 GB
Boot ROM Version: 230.0.0.0.0
SMC Version (system): 2.2f44
Sudden Motion Sensor:
State:    Enabled
I'm trying to pinpoint what the holdup is, since in the past it was able to do facetime well.  Looked at it in Activity Monitor, but couldn't find any one culprit.  I copied the console logs from that hour, and I'll look through it for more info.  Just wanted to get the question started.
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.