I don't mean the romsets, those load a lot after. The common distribution for snaps, flyers, manuals, PCB, etc. are Torrent7z (7z solid compressed files). So in order for a screenshot to show up on a frontend it has to read the whole compressed file until it finds the archive, instead of reading the header then looking for an offset and decompress the image.
That's the reason I recompress those assets as plain ZIP files. The optimal would be to extract all their content on directories, but I hate having that many files laying around for fragmentation purposes.