For the best experience you should load as many leagues as possible
I make the top 10-15 European leagues playable, the top 5 or so African leagues playable, all the South American leagues, the top 5 Asian leagues, top 5 North American leagues. Only continent I do not load up anything for is Oceania (sorry!) All the other leagues I run as view only. But of course, the layout changes depending on what save you are running…If you are doing a North American save, you will load up every league from that region etc.
Download community made league files! Ignore the star speed layout, it’s super misleading - I run with about 150,000 players loaded and half a speed star. Goes fast enough for me, maxes the realism too.
I’ve never tried running every single league with every single player loaded though haha. What was the actual processing speed like for that?