Here is a sample of how the VPN can affect the internet speed using iOS devices. Keep in mind there are a number of factors that will affect your speed for better or worse; this is only an example and the speed will be different from user to user.
Also, please keep in mind that when you’re connected to Hotspot Shield or any VPN service, data going in and out of your device travels through the VPN servers first before reaching their target. That's how internet anonymity is achieved and why it's normal to experience speed reduction from 30%-50% when using any VPN service.
With Hotspot Shield Free on iOS:
With Hotspot Shield Premium on iOS:
In case you have a free account, you can try a Premium account for free here.