Every league to view only does nothing to processing time or player count. It does let you view the teams/leagues/nations though. That is it. It does nothing at all to sim time. So not sure why you wouldnt want every league to viewable even leagues you dont care about. Playable is really only for leagues you want to manage in. Or leagues you want to have a full staff. Example other leagues/nations you compete in continental conferences. Even custom db is only for players and not staff. So if you force load all english players but england isnt playable. Once staff quits, sacks, retire they wont be replaced.
The most important thing is how you setup your database. 400k players is nuts. As like 300k+ of them will be awful and it will just clutter up the database. Think you can do national teams and top tier in every continent and that gets you around 70-140k players. But most importantly all of the national teams will be real players and not grey players. All the other players are insignificant and thats why you dont want to load them all especially in long term save. Your system will choke. To me you really want all of the national teams filled out with real players cause those are the players that matter.
Full detail you use in your nation and maybe some custom detail on other continental competing nations. But if you are in America then you could still load up Europe players or the top 5 as playable to get the staff but set their detail to minimum. Cause it doesnt really matter cause you arent ever going to manage there.
Full detail chokes the processor more then players. I always use a custom detail layout depending on each save i am doing