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 Portable SSD T5 Review (500GB/2TB) - The Industry Standard | The SSD Review
Portable SSD T5 500GB | Samsung Singapore
Grab Samsung's 500GB T5 SSD for $80 at Best Buy | Engadget