Despite being an old question I stumbled here and I can see a lot of speculation here. The other answers are likely correct to some extent, but I believe it misses the crucial factor for the situation experienced by the asker.
VPN uses compression.
I'm using LZO-compression and that will improve my speeds up to 4x compared to the connection width. This is particularly noticeable using speed tests or when downloading/uploading easily compressible data such as csv-files. In my case this applies to openvpn Home-Office connections as well as to the service I'm using Private Internet Access.
Note that not all VPN-connections have compression enabled, however they all (should) have encryption enabled. Adding or turning on compression will have marginal impact on cpu-cycles and latency and possible improvement with throughput and large packet latency.
Requirements for the event to happen
- The VPN-tunnelling is active over the slowest part of the end-to-end traffic. Ie. two fast separate local area networks are bridged (connected) using a VPN. This VPN is active over a slower part of the network (internet).
- VPN-tunnel compression is ON
- The transferred data moving in the VPN-tunnel is not already compressed (such as picture files) or encrypted (sending eg. email)
- The speed is constrained by the bandwidth, not by the sending/receiving end