How much energy loss should there be in an internal hub 8-speed?
Anything that has been standing around for 10 years needs some maintainance before it can be used with confidence (especially if it was not maintained before properly). So if there is some extra friction it would not be a total surprise. You should check the manuals provided by Shimano what is recommended for your hub (https://si.shimano.com/#/en/search/Keyword?name=SG-S501&amp;name_web=). There it is shown that you take the hub apart and dip the inside into an oil bath. The maintainance interval mentioned is every two years or 5000 km. The manual also says that the hub is not completely water-proof. So maintainance is the only way to get rid of water possibly present in the hub.

Shimano does not seem to specify a temperature range for the Alfine. So it should be possible to assume that they work well in all temperatures that people ride bikes in. But it is also reasonable to assume that whatever lubricant is in the hub will become more viscous when colder, resulting in less efficient lubrication. And again: the hub is not completely water-proof, and if you have a lot of water, that can definitely freeze.

That said, I reactivated a bike with a Shimano Nexus hub that has been unused for about 5 years for my wife, and it was no problem at all, although I skipped my own recommendation from above.  

One personal comment on internally geared hub vs. derailleur gears: I changed from derailleur to hub two weeks ago and to me it is like a dream. But I am lazy with cleaning the drivetrain, so what I usually use is far from being friction-optimized.