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VPN Bandwidth question

I have multiple remote sites connected via VPN tunnels using 3des and on the process of investigating latency that we experiencing over our network. I am using Solarwinds WAN killer tools to send packets of 1500 byte size using udp echo (7) to each remote site to identify how much traffic is needed before the connection slows down. Base of my test, I notice that when the amount of traffic reach between 40 - 46% of the bandwidth, latency starts to happen. In example, for a 2 Mbps bandwidth once I start sending 46% of its total bandwidth (approximately >920kbps) to the vpn tunnel, latency starts to happen. However if I lower the traffic to just 45% of the total bandwidth (>900kbps) no latency is experience. I also observe that sending un-encrypted traffic doesn't show any latency at all even if the total amount of traffic is over 46% or even 150% of its total bandwidth. Base on this observation, I think passing encrypted traffic between 40 - 46% of its total bandwidth seems to saturate the connection pipe. I just wonder how much traffic is really produced within the tunnel. Is there a known rule as far as how much actual bandwidth is achieved when using vpn connection? Thanks, Nick S.

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