Facebook chat lag with specific person.?

[Konstantinos]

My facebook chat lags with only one person. Started few months ago and stayed that way.
I have to refresh the page everytime to see if it has actually send my something, same thing happens with messenger on android. It's like only the chat between us two is bugged.

[Hockey]

.

[J]

If you have tested with others and it only seems to happen between you and the specific person, it is highly possible that person's internet connection end is bad/low.
Also data tends to travel through multiple routers, servers, and switches before arriving to it's destination. So the other possibility is that whatever route that data is taking could be a slow or poor one.
If possible, I would suggest you contact Facebook about it to see if anything is up with them. After all, it is their interface.