@Wolfieee
If the first start is slow and it isn't your NAS - then try to use subfolders for the ROMS. I made a test with RetroPie 2.3 in former times.
10.000 ROMS for 3 systems, and the Pi was up (from the terminal to a manuel starting ES) in 7 seconds
I put 18.500 ROMS to annother system (so 28.500 files in total), new the Pi needed 2minutes and 50seconds to start the ES from the terminal bash.
I divvided the roms in Subfolders A-Z and every subfolder was marked as a new system the starting time was 28 seconds :)