The bug occurs when I open my friendlist window (social) and choose a person to whisper to. That opens a window in which a conversation can be seen. Once the message is sent, game fps drops to absolute 0, preventing from playing for about 5-10 seconds, then it goes back to normal. I have also noticed that receiving messages while having the social window open at the person I'm messaging causes the same problem. Strangely enough, whispering via chat, /w <nickname> works just fine.
I spoke with some people about that issue, some have it, and some don't. This is why I am adding a pool to check how many people are bothered by this bug and whether it's something minor or something that should be looked into.

I am playing on EU server. and would appreciate some comparison with other servers.

EDIT: 19th Feb, this seems to be fixed now, it bothered me for 4 hours yesterday (re-launched game twice to check if it's just some minor problem) and now it is gone. Not sure about other people though, leaving this topic for other people to vote now.