The VPN technology will decrease your actual speed as ClearVPN applies encryption to your data and routes it through a server that may be on the other side of the planet. So, your VPN speed will depend on your location, the server (how far away it is), and the used protocol.
If you activate the Secure browsing's Balanced or Top protection presets, ClearVPN will connect you to the closest server. This way, your speed will be less affected by the distance, and you shall expect no more than a 15-25% drop in speed.
If you connect to the farthest server with a Location changer, your speed may drop by 25-70%.