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
Die besten Festplatten - Vergleich: SSDs und HDDs für PS4 und Xbox One
PS4-Geschwindigkeits-Boost mit SSD-Festplatte - G-Zockt.DE