Bungeecord is lagging with 400+ players

I am using 1 Bungeecord instance and with 400+ Players it starts lagging, but the CPU is only used 20% by Bungeecord.

I am using 1 dedicated server only for Bungeecord:
CPU: i7 4770k
RAM: 16 GB
SSD: 120 GB
1 Gbit/s FFE
OS: WinServer 2012 R2