I installed Kurento 6.0 one2one video call sample on my local machine and tested in local network. Chat worked smoothly. After that I put it on my dedicated server and tried it over internet. It starts to lag and freeze 10 or 15 seconds after the start of call. The server has 16mbits download speed and 10 mbits upload speed. It seems enough for one conference.
And what's your connection speed? Please check also the CPU load, as if you've chosen a very small instance, or using a computer vision filter, you might find that it's eating up all your resources.
EDIT 1
It looks like the link quality of your client is not very good, and the lagging is not in the server, but in the client.