Why does FaceTime slow down my phone?

Whenever I call someone through FaceTime or FaceTime Audio, eventually my phone begins to lag. For example, whenever I try to open the multitask screen the phone lags and takes a while to open it, and when I try to navigate through my open apps the phone lags as well. Or when I try to open the home screen the phone lags and takes a while to do so. This happens every time I call someone. This is all happening on the iPhone X since I got it this year in April.