Since there is no way to make the game bug proof yet, (and gog doesn't appear interested in trying any harder to make it so,) I tinkered a bit and found a way to fix lag in a way that works for me and I hope it may work for you as well.

After trying suggestions from the forum, here, someone said something about the game running well when put into resolution compatibility mode, but still not playable. (the toolbars took up more space than the game window) I thought to manually go into my advanced display settings and change my resolution to 1280 X 720 - simple as that.

Well, in my testing, from what I can tell, DWM (the, hardware accelerated "Desktop Window Manager") is the cause if the lag. In windows 7 and Vista it was as simple as changing the theme or checking a check box under the compatibility tab to disahble. In windows 8 and later, there's no official way to disable.

There is a way to disable DWM in windows 8 and 10 but the results are.... less than desirable. Though, as soon as it's disabled, the lag disappears.

Sadly best usable workaround is to either shrink the window size or change the screen resolution to something lower as you had suggested. Or fire up a XP VM with virtual Box or something, as crazy as it sounds, the game actually runs faster in a XP VM than on Windows 10 using real hardware!