Editors of The Verge tried to test slowest possible NVMe SSD drive in PS5 which was ADATA XPG Gammix S50 Lite with read speeds 3900MB/s and there was no difference in load times in PS5 exclusives like Ratchet & Clank: Rift Apart
https://www.theverge.com/2021/8/4/22608153/ps5-ssd-speed-test-storage-expansion-m2-playstation-5We compared the S50 Lite against both the PS5’s internal SSD and a 5,000MB/s Sabrent Rocket 500GB in a battery of tests, including load times for games like early PS5 showcases Ratchet and Clank: Rift Apart and Spider-Man: Miles Morales. We also measured how long it takes to back up games to each kind of drive, throwing in a Seagate external HDD for comparison, since Sony now allows you to archive PS5 games and play PS4 games from an traditional hard drive.
The verdict? Surprisingly, even the slowest compatible SSD we could find had near-identical load times to the one Sony includes in the box. Sometimes it’s a second slower, sometimes a second or two faster, but basically it’s a total wash. Hopping through dimensions in Ratchet and Clank: Rift Apart, I saw no appreciable difference with the slowest SSD.