Low Performance in 2D Classic

I couldn't get any answer in technical forum, so I'm posting my thread here.

I'm a fan of 2d classic and I don't like to play the game in 3d views; but for this time (for this save) I guess I loaded too many players and 2d classic is annoyingly lagging. So I decided to try 3D and surprisingly there were no lags (if I don't tick "show crowds" option). Yes my system is not really good anymore, it's an old one but firstly, in my previous save there were no lagging in 2d classic and secondly, how exactly 3d is not lagging but 2d classic is? And I tried to play the game in full-screen (I usually play in windowed mode) and in full-screen, both in 2d classic and 3d, there was no lag; but computer is rebooting itself after ten minutes in full-screen.

I started to use gamebooster but it doesn't really change anything. I know I want an impossible success from my system but I don't understand why 2d classic is lagging while 3d is not (is this because SI doesn't work on 2d classic anymore? Do they?). I don't want to finish the game, I don't want to play in 3d and I can't play in full-screen. I guess you can't suggest anything about laggings but what about full-screen problem?