dpbeaton, thanks for the reply.
sorry for so late reply. I had been on holiday.
had installed http server on one vpn client. and done the test on the vpn link.
result is same, slow upload speed 🙁
One important point that I missed to express is Zeroshell runs on VIRTUAL MACHINE.
If I use Ubuntu on Virtual machine and use 3g, upload speed is not limited.
I wonder wheather the difference between ZS and Ubuntu kernel, cause such a issue?
I tested VMware, Virtual box with ZS, result is limited upload speed.
I would really appricate your help, I am still searching for a solution.