OpenVPN connection stucks after new connection established

Hi,

I've installed OpenVPN using the tutorial provided by Linode.

http://library.linode.com/networking/openvpn/centos-6

The OpenVPN works great. However, I find a weird phenomenon. That is,

1) Once connected to OpenVPN, the whole Internet connection will be blocked for about 1 minute.

2) After other OpenVPN users connected, all connected OpenVPN users will be blocked for about 1 minute. And all OpenVPN users regain connections at the same time.

I followed Linode tutorial step by step, and I did the setup on fresh CentOS systems (both Linode and one other VPS service). Could anyone please provide me some thought to fix the issue?

Many thanks.

Yadon