I'm afraid the answer is, it depends!
There is a very slight overhead when using a secure encrypted VPN tunnel, however, in tests, we have found this to be negligible when compared with ebb and flow of the wider Internet bandwidth, for example local ADSL/Cable contention, or inter country contention etc.
By far the biggest issue is the fact that when connected to us, ALL your traffic is going via our server which is probably in another country and possibly the other side of the world from you. For example, if you are in Australia and connected to our service in the UK, and then say download a file from Singapore, that file is going from Singapore, to the UK and then back to Australia, which will make a big impact on the throughput.
If however, you are connected to our UK service and download a file from the UK, you should see almost no difference in the throughput.
The other significant factor is the bandwidth and usage between your location and the country you are connecting to. For example, Singapore has very good Internet connections within the country with most homes having access to a 100Mb connection, however, all those fast Internet connections quickly saturate the links from Singapore to the rest of the Internet and as a result, connections from Singapore to Europe vary greatly depending on the time of day.
This is also true of many countries, where the local Internet connections are good, but the links out of the country are over utilised.
If you would like any more information or wish to discuss this further, please send us an email to firstname.lastname@example.org and we will do our very best to assist you?