HotSpot Shield is a VPN. There is no limit and I’ve actually noticed very few slowdowns with it.
Whenever you connect to the internet here is the way your are actually able to view a webpage. Your computer downloads the page from the internet and then you view it. Thus any time you load a page you are technically downloading something to your computer.
It’s actually a little bit more confusing than that, but for understanding how this related to the 10GB limit I hope this is sufficient.
Please let me know if you have any more questions.