Sorry to some few are still having issues.
It needs to be noted that 'Gigantic' , with many factions, planets, etc, are going to be resource intensive. It is important take that into account, knowing the specs of your own PCs, when you are judging performance choosing such game types.
For the people confident in their specs, there are two things I want you to try.
1) Virtual Memory:
Go here and look for the 'Page File Size' entry:
For testing this issue, it is CRITICAL that you set is as shown - to:
'Automatically manage page file size for all drives'
It will require a reboot. Try the game after.
If that fails to improve things:
2) Windows Defender:
I would like to have you try excluding the folders that GC3 uses from Windows Defender. If you use another AV product, please use the folders provided to perform a like action.
Go here to see how to exclude:
The folders to exclude would be:
\Program Files (x86)\Steam\steamapps\common\Galactic Civilizations III
Please let me know if this improves your load times.
Thanks for your patience here.
Stardock Customer Service Manager