Is it good idea using Java to implemente VPN ? I worry about the VPN-X performance

Yes, we think this is not bad idea. Because the CPU speed is increase so quickly that more CPU idle time is a problem. VPN is solution for Internet access, and network(WAN) speed is the real bottle-neck.

VPN-X is not a pure Java program, it need C code to access OS resource.  So the performance of VPN-X is very good, even beyond some native C VPN solution.