I use RC4 to manage a lot of different ROM collections. For the ones with a few hundred/thousand files, RC is quite fast.
But my MAME collection is quite large (~30k files), and it takes hours (literally, more than 8 hours) when I launch a fix on the whole collection. The parts of the process that are slow are the "Remove useless files" and "Remove useless roms" fixes. These take a really long time, and the computer doesn't appear to be doing much during that time (max 15% CPU usage, no disk activity).
My MAME collection is in ZIP format, I have a 4 core (8threads) CPU, 16GB of RAM, and fast SSDs