This is the beginning of Google's push to rank sites that are mobile compatible and give them more weight. (I'll be writing an article on this in the next few days.)
Having said that, it's also a function of your hosting company. My store ranks in the top 81% of all sites anywhere, for speed of page loads. That is because of the hosting company I use. The speed at which the store loads on computer or mobile device is most dependent on two things, the speed of the server and the connection speed of the user.
Additionally, you can't always tell when someone will clear their cache on their machine, many folks have their browsers set to clear it every time it closes. So, while it'll make some folks load faster if the cached images are dated, most folks won't notice a difference.