A video call is unusual among everyday internet tasks because it sends as much as it receives. Most home connections are built for downloading, so the upload path is much narrower. When that narrow path fills up, your picture freezes for other people long before you notice anything on your own screen.
Steadiness matters more than raw speed. A call needs a small, continuous stream of data arriving on time. If packets arrive in irregular bursts — what engineers call jitter — the software has to guess at the missing moments, which is heard as robotic audio and seen as blocky video.
Wi-Fi adds its own variability. A laptop at the edge of coverage will keep re-sending data, and each retry arrives late. Calls that are fine in one room and unusable in another are almost always a coverage problem rather than a broadband problem.
Finally, the call software will quietly lower your video quality to protect the audio. That is why a call can stay understandable while the picture turns to mush: the app is already doing damage control.