Just bought the Samsung T5 500GB SSD and ran some speed tests against the PS4 slim HDD. Not scientific by any means but got some some interesting results. See comment below. Btw
Samsung 860 Pro 1TB SSD for PS4 Pro 評測│ 坂本Sakamoto.blog - 探究科技未知領域
Samsung 860 QVO vs EVO 850 Speed tests | Game Load | Copy Files - YouTube
PS4 Pro SSD Add-on Greatly Reduces Loading Times and Texture Pop-In
PS5運行PS4遊戲時,外置SSD的加載速度比內部SSD更快- 壹讀
FD 1TB PS4 SSD (Solid State Drive) - All in One Easy Upgrade Kit - Compatible with Playstation 4, PS4 Slim, and PS4 Pro (PS4-1TB-SSD) by Fantom Drives : Electronics - Amazon.com
We upgraded PS4 Pro with an 8TB SSD: can we make a better console? | Eurogamer.net
速度提升3 倍!教你將PS4 升級使用SSD!
10 Best External Hard Drives For PS4 2020 - YouTube