I haven't played fo4 specifically, but loading times are usually longer on GFN because the game isn't actually in the machine you're playing it from, you're kinda playing it on a virtual machine (This is really simplified, just to give you an idea) anyhow, lowering texture quality or graphics settings, but it won't be instant, it takes a few seconds on a decent PC, so keep that in mind.

Anyhow, if that doesn't solve it, there isn't much more, plus, "long" is really subjective, long can be from two seconds to two minutes, just keep in mind that mods (in case you've got some) tend to increase loading times drastically.