TERA, surely.
It's running on UE3, but I've played much smoother games on the UE3 engine with good graphics. They were not the same type of games of course, but TERA feels really badly optimized. There's a map called Cutthroat Harbor, and everyone in it (regardless of their systems hardware) won't get past around 30FPS or so, maybe 40 if you've got a GTX Titan or something along the line of a SLi or Crossfire setup. There's other maps where you can get maybe 50FPS, but you'd move some distance and you're suddenly in the low 40's again. That's pretty much without regard to the screen resolution too, or even in-game settings (even low settings don't help that much).
Additionally, the GUI is using Flash Player and many (including myself) report a significant performance decrease with the UI on (it can be removed in-game to take screenshots by pressing Ctrl + Z). That's the case for me, so when I go to a Banker and open up my bank's UI window my FPS goes from anywhere between 15 to 20. If I have both the bank's UI window and my inventory's window open at the same time I'm at a FPS crawl at around 10 or so (but also depends where I am located, it mostly happens in Velika, Kanstria and Kaiator, a bit less is Allamentheia).
That's at, or near maximum settings. Now the bigger problem is whatever I've tried (.ini values, and various other "fixes" posted all around the web about it) doesn't help much. Sometimes I do gain maybe up to 5 FPS with the so-called fixes, but it's usually negligible. I've also tried the absolute lowest in-game settings I could, and I still don't get past the 35FPS mark in most maps, with the occasional 45 or so when there's basically nothing going on-screen or very few mobs and no players happen to be around.
So, yeah, good game (I do really like it overall although it's nothing "mind blowing", it's just fun enough I'd say) but poorly optimized.