To determine your speed you first need to understand a few things: speedtest.net can be a good way to determine the speed potential, but only if used correctly.

1. What happens in speedtest.net is this; as you may have noticed before. Speedtest.net recommends a server based on the GEO location of your VPN. In many many cases this GEO Info is already wrong (unless you already live close to the server you're on) 

In order to do correct speed test and compare your own original speed with the VPN speed both tests must be started from the same server, this start server must be in the near of your original residence.

Lets assume you live in Madrid; do a test first of your original speed without the VPN, then manually choose Madrid as the starting point, or some other close server near you, Barcelona for example. Now you have your original speed potential.

To test the VPN speed now and being in a position to compare the results, the same server must be chosen as the starting point. If you chose Madrid, Madrid must be chosen again as the start server while testing the VPN!

Another good way to determine the general output speed potential of your VPN is the website http://cachefly.com, one of the fastest and best network test sites. Download the 100MB file and even better use this Download Accelerator http://www.speedbit.com/ or any similar one, too. This uses multiple connections to the file, which optimises downloads and shows an even more accurate full speed potential of the VPN, especially when you have a very fast internet connection.

As a final note: speed tests variate from day to day on some countries, not only this, doing several tests could give you different results. So one test is not enough, do a few of them and spread them over 1-3 days to see if there is any change.

2. You can as well simply download this link, its really accurate and gives a good overview of the real speed: http://cachefly.cachefly.net/100mb.test