Go ahead, load up just one of those images plus the rulebooks a player needs. FGC is well over a decade old, with networking architecture that is showing its age. FGC won't see any more major optimizations which is why they have been working on the Unity client. FGC also doesn't have an intermediate server. The link is between the GM and players only.
Did you load anything else with those images in your tests?
FGU is far from optimized at this point and they are still working on bug fixes for it.
Here are some screenshots of my FGC client with and without a theme. At the start I am using over 500mb without images shared, and you can see what I have opened in my library.
Add the combat tracker with enemies on it, a map with tokens...