There are different types of traffic and they have different performance characteristics. VPN throughput, due to burden of encryption and decryption, is very low in these units, approximately 1Mbps. Normal traffic like web surfing, email, etc is very different and much less burdensome, so throughput is higher for that kind of traffic.
In your case, you would want to be sure what your download vs. upload speeds are, assuming the connection is an asymmetric one. If the upload speed is greater than 1Mbps, then the 31xx won't be able to provide enough VPN throughput to fill the upload pipe. If you have greater than 1Mbps upload speeds and you actually want to take advantage of that with VPN traffic, you'll need to consider a higher-throughput unit like the 3430 with Enhanced Firmware.
Adtran Applications Engineering