Are you trying to run BG on a newer operating system or with a newer graphics card? I was having the exact same problem (slow frame rate) when I installed BG2 on my new laptop a few days ago. I solved my problem by turning off hardware acceleration. It's possible that my solution could also solve your problem. http://www.gog.com/en/forum/baldur_s_gate/bg2_frame_rate_problems_on_windows_7
The second post in that thread explains the step by step process for turning off hardware acceleration. It's kind of annoying, but I think it has solved the slowdowns. Try it out and let me know if it works for you.
NOTE: This was a fix for BG2, I have no idea if it would also fix your problems with BG1, although it's worth a shot.