When I was still in the USA and the person who was my boyfriend was here in the UK, we tried Skype to be able to talk for free over the internet.
He was on broadband, I was on dial-up, and I guess my horribly slow dial up speed made the difference because we found there was a frustrating time delay of sometimes as much as three seconds, before what I said to him reached him, or what he said to me reached me.
Two fast connections, however, may not experience that time lag. But it was so bad for us that communication was never smooth and we wound up going back to phone calls instead, especially for important discussions or information exchange.