I was just playing around a bit the other day and opened the Windscribe Pro VPN, installed a new version, and wanted to see how it affected my internet download speed on wifi. As expected, it slowed my connection speed down some, and this is the price one must pay for the security from eavesdropping and hacking when on a wifi network, especially a public wifi network.
Now, I don't need a VPN on my home network -- I was just playing and checking things out. As part of this, I checked several speed test websites, which I list below. Resulting download Mbps varied considerably between the different tests. Which was more accurate? Darned if I know, but the speed of my accessing the internet is acceptable, so I don't worry about what the actual speed might be.
In case you might want to do some checking yourself, here are the websites I used.