First of all, hello!

Your game is awesome, and I was loving it until I made the (arguably bad) decision of upgrading my windows from 8.1 to 10. Now the game is way too slow... It starts to lag IN THE MENU SCREEN!!!

I tried everything I could find in the forums: Running as 32-bit, running as admin, lowering every single video setup, running as win 7 (in the compatibility tab...), unninstalling and reinstalling... Nothing fixed it!

My specs:

Intel Core i7-5500U 2,40GHz
16 GB RAM
AMD Radeon R7 M260

(I know they are not the best, but with low quality video settings it was running very well...)

Please help me, I'd love to play this game again...

P.S:

I haven't included any files because I'm not sure which are relevant to my situation (it's not crashing, and the problem is not with an specific save file)

Perhaps your drivers are not "up to date" ?

http://support.amd.com/de-de/download/desktop?os=Windows+10+-+64

Is this a constant lag, or does it occur about 1/3 to 1/2 of the way into the game?  I have an issue with bad stuttering that only occurs on big maps with 8 empires, but I believe that is from my dual-core i5 @ 2.5o GHz.  If I open my task manager and resource monitor, I can see the CPU usage peaking being consistent with the stuttering.

I can't think that just upgrading to Win10 can have such a negative effect on GPU performance :)

Maybe Win10 is not using your R7 M260, but the HD Graphics from the 5500U instead ?

Can you have a look at this thread : http://www.tomshardware.co.uk/answers/id-2318050/amd-radeon-m260-working.html - maybe the R7 is not enabled, or not set to max performance ...