FPS drop caused by portraits

[NotScias]

Hi,
Since the CoR update I'm experiencing a strange FPS drop that only happens when I have someone (ally, enemy but not mobs) selected.

My FPS is most of the time capped at 60 (VSync) but once I select someone in the same situation I get immediately 10-15 less FPS at least. The FPS get back to normal immediately after deselection.

Here's some screenies taken at the same time to illustrate the issue :
--- Example 1
- No selection : http://wstaw.org/m/2012/12/28/fpsdrop00.jpg
- Selection : http://wstaw.org/m/2012/12/28/fpsdrop01.jpg
--- Example 2
- No selection : http://wstaw.org/m/2012/12/28/fpsdrop10.jpg
- Selection : http://wstaw.org/m/2012/12/28/fpsdrop11.jpg

I guess it has something to do with character portraits, since selecting mobs doesn't cause the FPS drop, also because sometimes the portraits flicker or are blank, causing the FPS to drop even more. It may be Linux/OpenGL specific since I asked some people running Windows to confirm the issue but they don't have this one. It happens regardless of the selected player, so no matter who is selected the drop will happen. It's more noticeable when the computer is more stressed (lot of people/fights), the drop is more consequent in such cases.
EDIT: It seems to happen after some minutes/hours of gaming. I don't have the issue right after logging in.
As for solutions I tried, checking/unchecking animated portraits doesn't help (I have them static all the time by default). I also tried to redownload all the ressources to be sure that it isn't texture corruption but it didn't help either. Drivers are up to date and I have absolutely no issues in other games.

So I would first like to know if other people are experiencing the same problem (especially on Linux), and if so I'd like NGD to look into this because it's really annoying and power wasting.

Thanks.