I have sent the Discord guys some feedback, because it seems that on initial launch Discord can generate a lot of lag. If this happens to you, make sure to quit the chat server and log back in, it should fix the issue. It can also come back up, so please do the same thing (quit/relog). I suspect some cache issue is at work.
Also, I was told the overlay (the possibility to read text messages and see who logged on etc. while being in game and not have to alt-tab to the window) didn't work yet for LOTRO. A little patience and feedback is necessary.
Please share your feedback/comments here so we can help the devs make it better!