First of all, thanks for your support!
Our Vanilla server broke another record last week with more than 9.000 players online at the same time.
That's really insane!
2015/02/28 : 4.000 players online
2015/03/03 : 5.000 players online
2015/03/17 : 6.171 players online
2015/05/16 : 6.667 players online
2015/05/17 : 7.440 players online
2015/05/25 : 8.096 players online
2015/05/31 : 9.092 players online
2015/06/07 : 10.000 players online ?

Some improvements were done to minimize lag in Raids, Instances and Battlegrounds (even at 10K players). But, as more players enter in the world it becomes more and more difficult to optimize server performance, forcing us to decrease some quality parameters to handle the large amount of players while developers continue to work on further optimization.
We are aware of this issue that some players have and a solution is on its way.
Best Regards,
Nostalrius