The VPN throughput seems slow, what can be done to increase the speed?

The maximum throughput you get over VPN is about 600Kbps (bits per second) when using 3DES. Try using DES as the encryption method, this will give you about 1Mbps throughput. If you need more than 1Mbps of throughput over VPN tunnel, you will need to use a product such as the RF660VPN, this model will give you VPN throughput of 15Mbps.