Unfortunately, Skype doesn't allow to control the quality of the video, tuning the connection by degrading the amount of data in the video stream. The program "ooVoo" allows doing that.
But the described use case is not your problem as you've fully switched video off.
I would suggest in your case revising the health of your system generally:
do you have enough free RAM?
enough space on hard-drive?
what is the network traffic and how much is that in comparison with your maximum?
Actually, the problem could be if those not too smart guys still trying to fight Telegram, destroying the Internet in Russia.
You know, that download speed could be very different if you connected to some domestic server or to the overseas servers.