My base is always the 12 following nations: England, France, Germany, Spain, Italy, Holland, Portugal, Scotland, Poland, Austria, Belgium, Brazil. Then, depending on which team I start, I choose particular divisions, usually I end up with loading around 18 leagues from 12 nations and 60k players, plus medium database. Thanks to this my game runs smoothly.
I never use any custom files (except the real name fix), never add/remove leagues during my save, and always save my game manually with multiple slots. Thanks to this, I never really have any crashes.
When FM 23 will be fully patched I plan to start my first ever long-term journeyman save that I aim to play for about 30-40 seasons, and I would like to load a couple of more leagues there. Do you guys think that if I choose, for example, 25 leagues from 18 nations the game will be much slower?