VPN performance in a 3120 would in the best of circumstances be about 1Mbps, more typically in the 700kbps range. 300kbps seems a bit low but honestly it depends on what else the unit is doing at the time (CPU utilization is an issue obviously) and how the unit is programmed for things that would slow it down like ACL's, QoS, etc, etc.
Based on your cable modem connection speeds (35/5), the 3120 is insufficient for that connection anyway. We don't generally recommend the 3120 for connections greater than 10Mbps. For a 35/5 connection, with the expectation to actually be able to fully utilize that bandwidth, we would recommend a 3430 unit, with the Enhanced Firmware for the VPN functionality.
Adtran Applications Engineering