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Do you notice this slowness right at the start or are you several years into a game. If you are quite a way into a game then the slowness is inevitable im afraid as the size of the game would be huge after a few years. If this slowness is right at the start then have you tried going into task manager and disabling as much as you can or checked to see exactly what is using all the memory.
Virus scanner? Put FM to its exceptions if you have a heavy virus scanner. Put the detail level on minimum. The game should be fine on your pc, but it is still a heavy processing game. Also, make sure you select 'threading' in the preferences menu so it uses hyperthreading.
Did you mean you retained players&staff from 6-7 countries, or just that you selected the leagues as viewable? If it's the former then I think that's the most probable reason it's slow. That's an enormous amount of people which the game has to keep track of.
Also, when you add leagues from another continent you also get a lot of extra cups added that the game will have to calculate. I.e. if you add Brazil, not only will competitions in Brazil be calculated, but also Copa Libertadores, copa sudamericana, etc.