Just installed this again from GOG and after getting rid of the loading issue by modifying archives, the game runs as expected - except for one thing -

Horrendous mouse lag in menus and dialogue. It's not every single menu though, the map and main menu work fine, but every other menu is affected. Itīs like it runs at 5 FPS The actual gameworld works fine

System is Windows 10 x64, GPU is GTX 660

I've tried windowed mode, editing .ini, replacing cshell, different video options and compatibility/administrator options on executable