It's possible there is a problem with your BGT installation being biffed up but I think the problem is really with DirectDraw, the graphics API used by BG2 which has been deprecated and is very poorly emulated in Win8.1. I get abysmal performance on this old game unless I tweak some stuff first.
Here are some possible workarounds:
Force Optimus to use the nVidia GPU exclusively for whatever main .exe is used by BGT whether it's baldur.exe or BGmain.exe or whatever (I don't know offhand which it is for BGT). Be sure to use the correct path to the .exe
If that doesn't work you'll probably need to either enable or disable 3d acceleration and/or 3d animations. Some people have better performance with 3d acceleration enabled but can't get a proper fullscreen mode to work. For others 3d won't work at all.
a. Try installing Aqrit's ddwrapper. This can be finicky to set up though as there are several settings and it can be difficult to find what works. For this you may have to disable 3d which means you lose the smooth fog-of-war and some spell and sprite effects but get significantly better performance in exchange.
b. In my case I have to use the above ddwrapper in addition to DxWnd. I have to set DxWnd to "Compensate flip emulation" to fix cursor flickering but that doesn't work without ddwrapper being installed with all settings set to zero for some inexplicable reason. I only discovered this by accident.
Install Windows XP in a virtualbox virtual machine and install BGT in that. I've done this too and it has the best performance on my laptop but it's a bit of a pain to get fullscreen to work correctly. However it provides the best, most authentic original Baldur's Gate experience on my laptop.
Even though I got everything working well enough I ultimately threw in the towel and got the enhanced editions of BG1 and BG2 and TBH I'm very glad I did. New content aside the enhancements to the actual rendering engine has been more than worth it, IMO, even if many the good old mods don't work.
Good luck though.