Having performance issues with your UI(particularly combat scenarios)? Read here.
 

Alongside the usual technical forum posting, I've decided to put this here as well.

Has anyone noticed that in combat situations, performance can drop significantly - yet if you toggle off the UI, you gain the performance back?

I have confirmed that some .tga files can be the culprit. I don't know for sure, but suspect it may be that either the file is corrupt, or that it was poorly built - or maybe just a bad conversion to .tga
To give you a personal example : I was using an invisible.tga file, which is very common in UI builds these days. It happens that the one I had was causing me to lose half my performance in combat situations.
I swapped it for a copy from a more recent UI build and the problem has gone away.

Hopefully this doesn't happen to many users, but for those that end up googling something like " LOTRO UI choppy combat" here's some tags just for you :

LOTRO UI User Interface choppy performance combat