The whole point of a VPN is to make any device less vulnerable to threats by providing an encrypted private connection. However, when using any third party public VPN, the question is, how well do you trust the VPN provider?
Exactly.
Beyond that, no, a VPN should not affect the actual functionality of the device, but depending on their own infrastructure it may or may not slow down your internet access. As you are routing all your internet traffic through the VPN provider's servers, if they are not up to the task it may slow down your browsing considerably.
The question is, why do you believe you need a VPN at this point? Why are you using one? If you do not trust the VPN provider to keep your data secure, then there's no benefit in using a VPN.