Having not been in a large clan for a while I hadn't noticed this for a long time
However, having recently played on an account in Epoch I can say that it is still there in it's full closed beta buggy glory
On mid-lower end computers, the game experiences a small amount of slow-down each time a clan member does something that would send a notification (turning them all off completely via unchecking AND checking "disable all" does not fix this). When not in a large clan this is not an issue, although playing on an account in Epoch I can safely say that the game is unplayable at peak hours of activity due to it essentially feeling like I'm running a CPU-intensive task in the background (unzipping, video rendering etc all cause the same effect, surely an in-game UI shouldn't be causing the same level of slowdown as vegas/live 8 )
Currently logging out from chat via console stops the problem, although this is not an ideal option as it means you can't access any hero guides in-game as well as being auto-logged out after each match on top of not being able to communicate with anyone inside HoN who isn't in your match whilst playing - not ideal.
I have no idea of the technical reasons for this but I'm hoping I've just been silly and missed a hotfix somewhere when trying to find if this has already been reported
EDIT: I know this is to do with notifications because if I enable them, the slowdown corresponds to them popping up on screen. Forgot to add that.