@ricebow *notice inb4 made a lot of typo's. I have been unwell a few days and have not been all clearheaded. Makes focusing and typing difficult, I address this since I will not fix the mistakes, got no energy but as someone who writes felt like I needed to adress it. (even if it is just for my own peace of mind) on to the essay. Hope the info helps, pic added for clarification on the last addressed issue.
New myself, but allow me to try and clarify, the lag is slowdown not lag, it is the same as when using file explorer where the file has 1000s of files. It slows down exactly to not lag out using cache. So far I have over 6000 games in now, and the slowdown stays the same as when it was around 1000s. Just the software limit is my assumption. Unlike file explorer (where as said slowdown happens too) launchbox itself takes ram and uses processes as well. So the slowdown seems more prominent. (I am not tech savvy by nature, but I always go through all kinds of documentation and forums). These are the conclusions I took after a week of setting up launchbox.
retroarch has next to everything, (via cores, also have found out some cores that can not be found within retroarch automatically exist out there, so potential to actually get next to everything proper, however some standalone emu's are preferred, (sometimes for accuracy, sometimes for speed, sometimes user specefied reasons) even on a game to game basis from time to time.
This is exactly where launchbox comes into play, you can integrate retroarch as well as the standalone versions of emu's separately. (you can add mednafen and other multi platforms into launchbox as well) Use the official tutorials made by ETAprime on the official launchbox channel. He goes over setting up fullscreen for basically any and all emu's in launchbox. Just look for the tutorial matching the emu you are working on. Some have settings, some need command line parameters. Can't find a tutorial for it, find some documentation on github, or some of the popular rom site choices, forums are a safe choice, and if all else fails, there are proper obscure rom sites with documentation. Google is your friend in this, since I do not know the stance on linking things.
Missing media is from misspelling on the original rom uploaders part, as well as some teams putting the number id infront, or putting suffix at the end and forgetting to use bracket (which launchbox will ignore. If the name of the game is not the same as it is on the database (meaning the filename) then it won't find it. For roms that just have numbers in front or wacky stuff at the end you can us bulk rename utilities find a preferred one. Typo's you will have to deal with and manually edit the game within launchbox rename it to the correct name as it was officially released (it is cares about punctuation and grammar and everything) and look up in the database. (you can do it in file explorer but in the end both are manual. And doing it withing LB makes you able to immediatly get metadata and media after renaming.