The down speed is 1mbps and up is about 600-700kbps.
Your speed tests make sense, then. As your upstream becomes saturated with outbound connection requests, your downstream speeds with stabilize at a speed lower than what you might expect. This can also be affected by the vpn link itself using udp packets which are difficult to optimize (QOS-wise). If you have the inclination and are utilizing an OS that supports such things, you can look into a QOS method called ACK prioritization for the virtual device itself. Personally, the only operating systems I've implemented this on have been BSD/PF-based--I'm sure there are docs somewhere to do the same in Linux.
-Stephen
