All VPNs are going to somewhat slow the speed of your connection to any site because that is how the system works and it requires firstly a greater amount of data transfer from point to point and it requires the computing effort of deciphering the encrypted point to point messaging.
But in the recent times, it isn't noticeable as far as day to day browsing is concerned provided you're with a good provider. At least, that's what my experience is from using a VPN.
Sometimes, ISPs throttle our connection as well --boo!!-- hence I typically use a VPN to get a speed boost whenever I have to download big files as I wouldn't wanna wait till the sun sets lol.