At the moment it appears to me that Infinity Engine games will always use 100% CPU on one core, no matter what you do (menu, or in-game).
This is fairly common in a number of old games. Hoyle Word Games will sit there & eat a core and that needs next to no CPU. Some part of the code sits there spinning and spins a lot faster at 2 - 3 Ghz.
Sounds like something like that. There have been some similar cases where e.g. a static menu causes at least GPU working overtime because there is no limit how many times per sec the (static) screen is refreshed. I think in some cases forcing vsync on from graphics driver settings may fix that.
However, in this case the CPU core stays at 100% as soon as you launch the game, no matter whether you are watching the BG2 intro videos, or the menu screen, or actually are playing the game. The only time the CPU core usage percentage seems to go down for a second or two is when you are loading e.g. a saved game (ie. the game apparently concentrates on HDD activity).
Forcing vsync on in the driver settings didn't fix it at least for me, and the CPU usage stays at 100% even if you run BG2 in a window (in which case it obviously uses the same vsync settings and such as rest of the system, as I don't think a single window can have a different video card refresh rate than the desktop and other windows).
So you're an intransigent soul, ploughing on, blaming a game engine that runs perfectly fine on countless thousands of machines all over the world? That's fine by me; you asked the question, now get on with it.
Define "perfectly fine". I presume that 99% people are not watching Core Temp or some other similar utility while they are playing BG2. In fact, neither did I realize that BG2 is using 100% all the time (for one core) until I checked it with Core Temp application while running the game (in a window, as how else can I watch the core temp output simultaneously?).
So yes, BG2 runs "fine" for me too, even though it uses 100% CPU (core) for no good reason. I just happened to notice that my older laptop runs hotter than normally when playing BG2, and Core Temp confirmed why that is (BG2 is doing its own CPU burn testing while I am playing the game). The slightly more severe symptom was that _occasionally_ it got even so hot that Windows or the CPU controller started throttling the system all the way down due to overheating, which I'd see as jerky game for some time (until the CPU cooled down).
Whether it is due to Windows, the game itself, or video drivers, or some GOG wrapper, or the combination of them all, I don't know, obviously. I can already tell that other games I've tried don't constantly eat 100% of CPU core (e.g. Wheel Of Time), maybe I need to check more Windows 2D games like Diablo 2 etc.