I do believe it has to do partially with geographical location. The longest loading times seem to be when I play with my friend in Australia. That said, it still shouldn't take that long. There has to be something wrong in the game, because I'm sure my friend can upload the save somewhere else and I can download it in less than a minute but in the game it can take up to 30 minutes, like I said.
Also, both my upload and download speeds are above 10mb/s so the problem is not that. The problem is not with my computer either since I just bought it this year and it can run the newest games like the Witcher 3 and GTA 5 without any problems either in SP or MP.
And like Sirjojo said I searched on Google and I'm definitely not the only one with these problems: https://www.google.com/search?q=eu4 slow loading multiplayer
Have you both tried forwarding ports and turning off firewalls?