Fat PS3 was only capable of 1X - 2X Bluray read ~9MB/s while Slim and Super Slim were capable of 6X ~27MB/s (changes over the surface of the disc of course) so I don't think that's it. PS3 used a rather shitty BSD FAT32 driver for their 5400rpm laptop drives and SSD upgrades were better but still heavily bottlenecked by said driver.

I can load a decrypted ISO directly in RPSC3 over 1 Gigabit ethernet from a NAS with that inherent horrendous latency (compared to local storage drives, especially random access) and it'll still load faster than a game would on an actual PS3 on either or even combined storage mediums like many games partial installed to HDD and also used Bluray.