I can't think of any reason why a game like this should run so poorly. Sometimes it's not the computer's fault. Sometimes it's inefficient programming.
I was disappointed when I retired at the end and the unit I selected (a light infantry that I had from the start) was reset to level 1. Still, great game.
I don't like this game as much as your others because it seems like the same strategy with minor variations can be used for each level. But the game is still something to be proud of.
Why do developers think video walkthroughs are a good idea for adventure games? Why would anyone want to search through five minutes of footage to find the one item they missed?