My apoligises for having missed your post too.
An immediate suggestion I can give you, as Philippe did, is to deactivate all firewalls / antivirus programs. Some people reported problems very similars to yours, despite having a good PC.
Try also playing in windowed mode (use the Setup program you will find in the game folder). So, when playing in windowed mode, you can execute the task manager in order to check if there is a resident program that is "stealing" CPU...
If you indentify that program, you know the cause of GI loss of performance. Some firewalls/antivirus program, indeed, slow down the I/O and RAM access operations performed by the game.