The APU is certainly not slower than a PS4, it has Zen 2 cores. The GPU might have fewer flops but it has a much newer architecture, so it could easily be faster. Just comparing flops across different architectures isn't an accurate way of determining GPU gaming performance.

Do you expect this 15W APU to be faster than full TDP Desktop AMD Ryzen 5700G? The "Fastest Integrated Graphics Ever" 13 days ago https://www.tomshardware.com/news/amd-ryzen-7-5700g-review? 5700G reaches GTX760 parity in some games like Witcher3, while in others like Overwatch its medium for APU while dedicated gpu runs same framerate at Ultra.

It'll be interesting to see. RDNA2 is definitely a significant upgrade over Vega. Also I would assume most of that desktop TDP goes towards the CPU. The LPDDR5 RAM will also help the GPU in the Steam Deck.