There are three main factors impacting the internet speed through VPN service:
your internet speed
your location, and distance to the server – number of devices between you and the server.
location of the VPN server that you are using and the destination server (WWW, FTP, etc.).
Technically, a VPN will slow down your connection, even in the best-case scenario, because encryption partially depends on your device. Therefore, you should start with a trial/30 days money-back guarantee and test it on your devices.