Hey everyone!
I have some serious lag issues with this game. I'm getting anywhere from 10-25 FPS in the overworld and in battles, and 5-10 FPS in the menus (hero tab, spellbook etc). It's not like it's unplayable or anything, but it's really annoying me, especially because I remember playing this game way back on a slower PC and it ran butter-smooth.
Moreover, changing the graphics settings has no effect whatsoever, FPS stays the same both on maximum and on minimum settings.

Here are my specs:
Windows 7 Ultimate (64 bit) SP1
8GB ram DDR3
AMD Phenom II (3 CPUs), 3.4GHz
AMD Radeon R7 370 (4GB ram)

Any help would be apprecaited!

Well, that shot in the dark certainly worked, getting smooth 60 fps in the overworld and 45ish in the menus now, thanks a ton! The only annoying thing is that I have to set it to single core every time I run it, but it's not a big deal. Don't suppose there is a way to do it automatically through a shortcut command or something?
Strange thing is, my old config also used a radeon card and had no issues.

There surely is. You have to simply add a command into game's shortcut.
Follow this guide or this video.